Increase in catcalls by end of act (5)
I believe the answer is:
boost
'increase' is the definition.
(boosting is a kind of increasing)
'catcalls by end of act' is the wordplay.
'catcalls' becomes 'boos' (I've seen this before).
'by' means one lot of letters go next to another.
'end of' suggests the final letters.
The final letter of 'act' is 't'.
'boos'+'t'='BOOST'
'in' acts as a link.
(Other definitions for boost that I've seen before include "Help to increase" , "Give more impetus to" , "Fillip" , "promotion" , "Push up (eg morale, chances)" .)