Bygone leader wearing musty uniform (6)
I believe the answer is:
stable
'uniform' is the definition.
(thesaurus)
'bygone leader wearing musty' is the wordplay.
'leader' suggests taking the first letters.
'wearing' means one lot of letters goes inside another.
'musty' becomes 'stale' ('stale' can be similar in meaning to 'musty').
The initial letter of 'bygone' is 'b'.
'b' put within 'stale' is 'STABLE'.
(Other definitions for stable that I've seen before include "It might accommodate Arab" , "Firmly balanced or fixed" , "Housing for a horse" , "farm building" , "container for punch, for instance" .)