Heartlessly glances back at toast (4)
I believe the answer is:
skol
'toast' is the definition.
(I've seen this before)
'heartlessly glances back' is the wordplay.
'heartlessly' means to remove the middle letters.
'glances' becomes 'looks' (I've seen this in another clue).
'back' is a reversal indicator.
'looks' with its middle taken out is 'loks'.
'loks' reversed gives 'SKOL'.
'at' acts as a link.
(Other definitions for skol that I've seen before include "Cheers in Copenhagen" , "Scandinavian toast" , "your health!" , "Scand. drinking toast" , "(Scandinavian) "bottoms up"" .)