Not a big audience though it was for charity (4,5)

'it was for charity' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'not a big audience' is the wordplay.
I cannot really see how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'audience' could be 'house' (term for a theatre's audience) and 'house' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

'though' acts as a link.

Can you help me to learn more?