A setter's retired to tackle good puzzle (6)
I believe the answer is:
enigma
'puzzle' is the definition.
(I've seen this before)
'a setter's retired to tackle good' is the wordplay.
'setter's' becomes 'mine' (what belongs to the crossword setter).
'retired' is a reversal indicator.
'to tackle' is an insertion indicator (some letters tackle or gain control of others).
'good' becomes 'g' (abbreviation).
'a'+'mine'='amine'
'amine' back-to-front is 'enima'.
'enima' placed around 'g' is 'ENIGMA'.
(Other definitions for enigma that I've seen before include "Mysterious puzzle" , "WW two Ger. code device; puzzle" , "Something mysterious and puzzling" , "Conundrum" , "Puzzle found in gem, Ian" .)