Hemp is a pound on Sunday! (5)
I believe the answer is:
sisal
'hemp' is the definition.
(I've seen this before)
'is a pound on sunday' is the wordplay.
'pound' becomes 'l'.
'on' says to put letters next to each other.
'sunday' becomes 's'.
'is'+'a'+'l'='isal'
'isal' put after 's' is 'SISAL'.
(Other definitions for sisal that I've seen before include "Plant fibre used for ropes" , "Natural fibre" , "Hemp fibre" , "Type of fibre" , "Plant yielding rope fibre" .)