501 pence for guacamole, say (3)
I believe the answer is:
dip
'guacamole say' is the definition.
(I know that guacamole is a type of dip)
'501 pence' is the wordplay.
'50p' becomes 'di' (50p in Roman numerals).
'pence' becomes 'p' ().
'di'+'p'='DIP'
'for' acts as a link.
(Other definitions for dip that I've seen before include "Depression in a surface" , "Go down" , "A brief informal swim" , "Slope downwards" , "Sink; brief swim" .)