Pro side (4)
I believe the answer is:
ayes
'pro side' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ayes that I've seen before include "Affirmative votes as in committee" , "Positive votes" , "Sounds of agreement" , "Votes of approval during 5 Down" , "The yes votes at committee meeting" .)