Flag officer on Western Isle (4)
I believe the answer is:
wilt
'flag' is the definition.
(I've seen this before)
'officer on western isle' is the wordplay.
'officer' becomes 'lt' (I've seen this in other clues**).
'on' says to put letters next to each other.
'western' becomes 'w'.
'isle' becomes 'i'.
'w'+'i'='wi'
'lt' put after 'wi' is 'WILT'.
(Other definitions for wilt that I've seen before include "Flag" , "fade" , "Lose strength and become limp" , "Decline" , "lose energy?" .)