Recruit learning about Navy gets upset (5)
I believe the answer is:
enrol
'recruit' is the definition.
('enrol' can be a synonym of 'recruit')
'learning about navy gets upset' is the wordplay.
'learning' becomes 'lore' (I've seen this before).
'about' indicates putting letters inside.
'navy' becomes 'n' (common abbreviation eg in RN Royal Navy).
'gets upset' shows that the letters should be reversed in order (I've seen 'getting upset' mean this).
'lore' placed around 'n' is 'lorne'.
'lorne' in reverse letter order is 'ENROL'.
(Other definitions for enrol that I've seen before include "Sign on, being a loner" , "Register (as student)" , "Sign on, register" , "Register officially" , "Add to list of members" .)