Workers betray defeat (4,5)
I believe the answer is:
shop floor
'workers' is the definition.
(I've seen this before)
'betray defeat' is the wordplay.
'betray' becomes 'shop' (I've seen this before).
'defeat' becomes 'floor' (e.g. knock to the ground in a fight).
'shop'+'floor'='SHOP FLOOR'
(Other definitions for shop floor that I've seen before include "Workers" , "part of factory" .)