Inca apt to upset an officer (7)
I believe the answer is:
captain
'an officer' is the definition.
(I know that captain is a type of officer)
'inca apt to upset' is the wordplay.
'to upset' indicates an anagram.
'inca'+'apt'='incaapt'
'incaapt' is an anagram of 'CAPTAIN'.
(Other definitions for captain that I've seen before include "Is Pat Cain in charge of the ship?" , "Skip" , "Naval commander" , "Skipper" , "Naval rank" .)