Fellow in military cap must be deprived (6)
I believe the answer is:
bereft
'deprived' is the definition.
(thesaurus)
'fellow in military cap' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'in' means one lot of letters goes inside another.
'military cap' becomes 'beret' (I've seen this before).
'f' placed within 'beret' is 'BEREFT'.
'must be' is the link.
(Other definitions for bereft that I've seen before include "Having had something precious taken away" , "Pained by the loss of someone" , "Deprived of or lacking something" , "Deprived, say of reason" , "Lonely and abandoned" .)