Quick snooze set up worker in close-fitting headgear (6)
I believe the answer is:
catnap
'quick snooze' is the definition.
(a catnap is a quick sleep)
'set up worker in close-fitting headgear' is the wordplay.
'set up' says the letters should be written backwards (in a down clue, letters go up).
'worker' becomes 'ant' (worker is a type of ant).
'in' means one lot of letters goes inside another.
'close-fitting headgear' becomes 'cap' (tight hat).
'ant' in reverse letter order is 'tna'.
'tna' inserted inside 'cap' is 'CATNAP'.
(Other definitions for catnap that I've seen before include "Brief time out" , "For a rest" , "Forty winks (for moggy?)" , "Brief doze" , "(Tom's) snooze?" .)