Unstabler sort of officer (9)
I believe the answer is:
subaltern
'officer' is the definition.
(subaltern is a kind of officer)
'unstabler sort' is the wordplay.
'sort' indicates anagramming the letters.
'unstabler' is an anagram of 'SUBALTERN'.
'of' acts as a link.
(Other definitions for subaltern that I've seen before include "Lesser official" , "Junior officer" , "Army officer below the rank of captain" .)