Study principally to be an officer (5)
I believe the answer is:
major
This might be a double definition.
'study principally' is the first definition.
'major' can be an answer for 'study' (thesaurus). I am not sure about the 'principally' bit.
'officer' is the second definition.
(type of military officer)
'to be an' acts as a link.
Can you help me to learn more?
(Other definitions for major that I've seen before include "Principal" , "Main course" , "Important; rank" , "Officer - of greater importance" , "Pre-eminent" .)