Fool to impersonate an army officer (4)
I believe the answer is:
dolt
'fool' is the definition.
(I've seen this before)
'impersonate an army officer' is the wordplay.
'impersonate' becomes 'do' ('do' can mean to do an impression of).
'an army officer' becomes 'lt' (short for Lieutenant).
'do'+'lt'='DOLT'
'to' is the link.
(Other definitions for dolt that I've seen before include "dope" , "Dullard" , "Dull or stupid fellow" , "Simpleton" , "Dunce" .)