Functional out of uniform fur (5)
I believe the answer is:
sable
'fur' is the definition.
(I know that sable is a type of fur)
'functional out of uniform' is the wordplay.
'functional' becomes 'usable' (having a use).
'out of' indicates named letters should be taken away.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'usable' with 'u' taken out is 'SABLE'.
(Other definitions for sable that I've seen before include "Black (heraldry); fur" , "Dark brown - black" , "Literary black" , "Adventurous courage" , "Bales of valuable Russian fur" .)