Limit placed on common use of straw hat in service (6,3)
I believe the answer is:
forage cap
'hat in service' is the definition.
(hat that may be worn in the armed services)
'limit placed on common use of straw' is the wordplay.
'limit' becomes 'cap' (capping is a kind of limiting).
'placed on' means one lot of letters go next to another.
'common use of straw' becomes 'forage' (straw is often used as forage on a farm).
'cap' after 'forage' is 'FORAGE-CAP'.
(Other definitions for forage cap that I've seen before include "Infantryman's headgear" , "Fore-and-aft military headgear" , "item of military kit" , "hat" , "Soldier's covering" .)