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'm not sure about the 'nicely' bit.
'when out of uniform to rest will' is the wordplay.
I cannot quite see how this works, but
'uniform' could be 'u' (phonetic alphabet: alpha, bravo, charlie etc.) and 'u' is present in the answer.
'rest' could be 'sit' (I've seen this before) and 'sit' is found within the remaining letters.
No letters remain.
This may be the basis of clue (or it may be nonsense).
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" .)