He keeps his uniform in a big box (6)
I believe the answer is:
curate
'he keeps his' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'uniform in a big box' is the wordplay.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' means one lot of letters goes inside another.
'a big box' becomes 'crate' (I've seen this in another clue).
'u' inserted within 'crate' is 'CURATE'.
Can you help me to learn more?
(Other definitions for curate that I've seen before include "Priest, a vicar's assistant" , "Clergyman assisting priest" , "Churchman" , "Take care of collection" , "Clergyman assisting a vicar" .)