Mature person shaving member of fighting force? (5)
I believe the answer is:
oldie
'mature person' is the definition.
(I've seen this before)
'shaving member of fighting force?' is the wordplay.
'shaving' means to look at the middle letters (the outside letters are shaved off).
'member of fighting force?' becomes 'soldier'.
The middle letters of 'soldier' are 'OLDIE'.
(Other definitions for oldie that I've seen before include "An octogenarian, perhaps" , "Aged person or thing" , "Aged person (colloq.)" , "The --, UK magazine" , "Perennially popular song or film, often golden" .)