Casserole reduced by a pound: a bargain (5)
I believe the answer is:
steal
'a bargain' is the definition.
('steal' can be a synonym of 'bargain')
'casserole reduced by a pound' is the wordplay.
'casserole' becomes 'stew' (I've seen this before).
'reduced' means to remove the last letter.
'by' is a charade indicator (letters next to each other).
'pound' becomes 'l'.
'stew' with its final letter taken off is 'ste'.
'ste'+'a'+'l'='STEAL'
(Other definitions for steal that I've seen before include "Take unlawfully" , "Pilfer" , "Snatch" , "Shoplift" , "Whip" .)