Topless shoot that's red hot (4,4)
I believe the answer is:
open fire
'that's red hot' is the definition.
(as in a red-hot fire)
'topless shoot' is the wordplay.
'topless' becomes 'open' (opened up and without a top).
'shoot' becomes 'fire' (firing is a kind of shooting).
'open'+'fire'='OPEN-FIRE'
(Other definitions for open fire that I've seen before include "Start attack" , "Order for military" , "Begin shooting" , "Domestic hearth" , "domestic hot spot" .)