Cloth worker becoming police officer (8)
I believe the answer is:
sergeant
'police officer' is the definition.
(I've seen this before)
'cloth worker' is the wordplay.
'cloth' becomes 'serge' (serge is a kind of cloth).
'worker' becomes 'ant' (worker is a type of ant).
'serge'+'ant'='SERGEANT'
'becoming' is the link.
(Other definitions for sergeant that I've seen before include "Pepper, say" , "Man in charge" , "one with stripes" , "military man" , "Rank above corporal" .)