Behead mollusc for pound (4)
I believe the answer is:
quid
'for pound' is the definition.
(UK slang for a pound)
'behead mollusc' is the wordplay.
'behead' means to remove the first letter (I've seen 'beheading' mean this).
'mollusc' becomes 'squid' (squid is a kind of mollusc).
'squid' with its initial letter removed is 'QUID'.
(Other definitions for quid that I've seen before include "Pound (sl.)" , "Pound (colloq.)" , "Pound (informal)" , "bit of bread" , "100p (slang)" .)