Rent, but not from Royal Navy (4)
I believe the answer is:
torn
'rent' is the definition.
(synonyms)
'not from royal navy' is the wordplay.
'not' becomes 'to' (I can't explain this - if you can you should believe this answer much more).
'from' means one lot of letters go next to another.
'royal navy' becomes 'rn'.
'to'+'rn'='TORN'
'but' acts as a link.
(Other definitions for torn that I've seen before include "Rent, ripped" , "Removed forcibly" , "Lacerated - in two minds" , "?Curtain (Hitchcock thriller)" , "Undecided between two" .)