Officer's English degree is specialised for an American (7)
I believe the answer is:
majored
'specialised for an american' is the definition.
(an American student majors in a particular subject)
'officer's english degree' is the wordplay.
'officer' becomes 'major' (type of military officer).
'english' becomes 'E' (abbreviation).
'degree' becomes 'd' (abbreviation for the degree in measuring angles).
'major'+'e'+'d'='MAJORED'
'is' acts as a link.
(Other definitions for majored that I've seen before include "specialized" , "Specialised in a particular subject" .)