Make officer a blockhead (4)
I believe the answer is:
dolt
'a blockhead' is the definition.
(thesaurus)
'make officer' is the wordplay.
'make' becomes 'do' (doing is a kind of making).
'officer' becomes 'lt' (I've seen this in other clues).
'do'+'lt'='DOLT'
(Other definitions for dolt that I've seen before include "Blockhead" , "Simpleton" , "Dimwit" , "Dunce" , "Halfwit" .)