Determined effort from exhausted group (4,3)
I believe the answer is:
dead set
'determined effort' is the definition.
'exhausted group' is the wordplay.
'exhausted' becomes 'dead'.
'group' becomes 'set' (set is a kind of group**).
'dead'+'set'='DEAD-SET'
'from' is the link.
(Other definitions for dead set that I've seen before include "Determined (upon)" , "Absolutely determined, bent on" , "Fully determined" , "A determined attack, sounds moribund" , "Absolutely - sedated (anag)" .)