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 certain of the 'nicely' bit.
'when out of uniform to rest' is the wordplay.
I cannot really see 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 in another clue) and 'sit' is present in the remaining letters.
No letters remain.
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" .)