He keeps his uniform in a big box (6)
I believe the answer is:
curate
'he keeps' 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' is an insertion indicator.
'a big box' becomes 'crate' (I've seen this in another clue).
'u' going into 'crate' is 'CURATE'.
'his' is the link.
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" .)