Some offer a reward — definitely not well done! (4)
I believe the answer is:
rare
'not well done' is the definition.
(as in a steak)
'some offer a reward definitely' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'RARE' can be found hidden inside 'offer a reward definitely'.
(Other definitions for rare that I've seen before include "Might steak be" , "Not easily come by like undercooked meat" , "like gold dust" , "few and far between" , "Still red inside (as hens' teeth?)" .)