Youngster in company with an officer (4)
I believe the answer is:
colt
'youngster' is the definition.
(I've seen this before)
'company with an officer' is the wordplay.
'company' becomes 'co' (abbreviation).
'with' says to put letters next to each other.
'an officer' becomes 'lt' (abbreviation for lieutenant).
'co'+'lt'='COLT'
'in' is the link.
(Other definitions for colt that I've seen before include "Male foal" , "Youngster" , "Early American revolver" , "Mount" , "pistol" .)