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 really 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 in another clue) and 'sit' is found in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
'will' is the 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" .)