Makes a special study of military men (6)
I believe the answer is:
majors
'men' is the definition.
(major is a kind of man)
'makes a special study of military' is the wordplay.
I cannot quite see how this works, but
'a' is present in the answer.
'special' could be 's' (common abbreviation as in Special Air Service) and 's' is found within the answer.
'of' could be 'o' and 'o' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for majors that I've seen is " Army officers".)