Time to wear a fatigue dress (6)
I believe the answer is:
attire
'dress' is the definition.
(attiring is a kind of dressing)
'time to wear a fatigue' is the wordplay.
'time' becomes 't' (abbreviation).
'to wear' indicates putting letters inside (to wear something is to be covered up by it).
'fatigue' becomes 'tire' (synonyms).
'a'+'tire'='atire'
't' inserted into 'atire' is 'ATTIRE'.
(Other definitions for attire that I've seen before include "'Dress, garb (6)'" , "Formal dress or costume" , "Formal clothes" , "Garments" , "Clothing, apparel" .)