Funny at first, prank involves a zealot (7)
I believe the answer is:
fanatic
'zealot' is the definition.
(I've seen this before)
'funny at first prank involves a' is the wordplay.
'at first' says to take the initial letters.
'prank' becomes 'antic' (synonyms).
'involves' is an insertion indicator.
The first letter of 'funny' is 'f'.
'antic' placed around 'a' is 'anatic'.
'f'+'anatic'='FANATIC'
(Other definitions for fanatic that I've seen before include "enthusiastic" , "Extreme zealot" , "One who is excessively zealous" , "Freak" , "Bigot - enthusiast" .)