The word "crowd" is singular; it refers to a group but the word is singular, therefore you use "was". In the sentence "a crowd of citizens was cheering", "crowd" is the subject and it is singular, so you use "was".
If you were referring to a group of multiple objects or beings (as that is what a group tends to be ...), say, a crowd of citizens, perhaps, would you use "was" or "were". e.g. A crowd of citizens was/were cheering? Cheers :)
Depends on the context. If you are talking about the crowd (the citizens being a descriptive adjective) then it is The Crowd was cheering.. If it is about the people in the crowd, then "The citizens were cheering.
Most of the time, you are safe with the standard convention that verbs are chosen by matching gender, number, and case.
In this case, if the word is singular, it is safe to use the singular form of the verb.
But the use of the plural may be used when you are trying to emphasize something else. For more information on that, go here:
http://blog.dictionary.com/collective-nouns/
Should I use a singular or a plural verb with a collective noun?
A collective noun refers to a whole group as a single entity but also to the members of that group.
A collective noun names a group of individuals or things with a singular form. Examples of collective nouns are: Faculty, herd, team. There are collective nouns for people, animals, objects, and concepts. The use of a singular or plural verb depends on the context of the sentence. If one is referring to the whole group as a single entity, then the singular verb is best: The school board has called a special session. When a group noun is used with a singular determiner (e.g., a/an, each, every, this, that), singular verbs and pronouns are normal: The team is away this weekend; they have a good chance of winning. There are other contexts where the plural verb is more natural: My family are always fighting among themselves. When the individuals in the collection or group receive the emphasis, the plural verb is acceptable. Generally, however, in American English, collective nouns take singular verbs. In British English, collective nouns are more often treated as plurals that take plural verbs.
