Terribly clean of body, for an officer (5,8)
I believe the answer is:
lance corporal
'for an officer' is the definition.
(lance corporal is a kind of officer)
'terribly clean of body' is the wordplay.
'terribly' is an anagram indicator.
'of body' becomes 'corporal' (I've seen this before).
'clean' is an anagram of 'lance'.
'lance'+'corporal'='LANCE-CORPORAL'
(Other definitions for lance corporal that I've seen before include "Junior NCO in the British Army" , "Army rank" , "Soldier" , "NCO rank" .)