Wag is huge, not small (5)
I believe the answer is:
comic
'wag' is the definition.
(wag can mean a comical person)
'huge not small' is the wordplay.
'huge' becomes 'cosmic' (on a cosmic scale).
'not' is a deletion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cosmic' with 's' removed is 'COMIC'.
'is' acts as a link.
(Other definitions for comic that I've seen before include "Children's periodical" , "Funny, laughable" , "Raising mirth" , "Professional joke-teller" , "Funny, droll" .)