When out of uniform, to rest will do nicely (4)
I believe the answer is:
suit
'do nicely' is the definition.
'suit' can be an answer for 'do' (thesaurus). I am not sure about the 'nicely' bit.
'when out of uniform to rest' is the wordplay.
I cannot quite understand how this works, but
'uniform' could be 'u' (phonetic alphabet: alpha, bravo, charlie etc.) and 'u' is found within the answer.
'rest' could be 'sit' (I've seen this before) and 'sit' is found within the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'will' acts as a link.
Can you help me to learn more?
(Other definitions for suit that I've seen before include "Set of cards" , "Legal action - hearts, perhaps" , "Formal attire" , "Formal wear for men" , "See 7" .)