Tax collecting? It's fun! (6)
I believe the answer is:
levity
'fun' is the definition.
(thesaurus)
'tax collecting? it's' is the wordplay.
'tax' becomes 'levy' (I've seen this before).
'collecting?' indicates putting letters inside.
'levy' placed around 'it' is 'LEVITY'.
(Other definitions for levity that I've seen before include "Flippant attitude to serious matters" , "Unseriousness" , "Silliness" , "Playfulness" , "Flippancy" .)