A couple of pounds and that's the lot (3)
I believe the answer is:
all
'that's the lot' is the definition.
(I've seen this in another clue)
'a couple of pounds' is the wordplay.
'couple of' shows that letters should be duplicated.
'pounds' becomes 'l' (L can represent the monetary pound).
'l' doubled is 'll'.
'a'+'ll'='ALL'
'and' acts as a link.
(Other definitions for all that I've seen before include "Garden passage between trees or bushes" , "Avenue, walk or garden path" , "without exception" , "The works" , "Fully clad ..." .)