When you've only a pound left, it means everything! (3)
I believe the answer is:
all
'everything' is the definition.
(all things)
'when you've only a pound left' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'pound' could be 'l' (abbreviation e.g. Lsd - pounds, shillings, pence) and 'l' is located in the answer.
'left' could be 'l' (common abbreviation) and 'l' is located in the answer.
No letters remain.
This explanation may well be incorrect...
'it means' is the link.
Can you help me to learn more?
(Other definitions for all that I've seen before include "and 16 Across and 23 Across: Even so" , "Avenue, walk or garden path" , "Passage in a formal garden" , "The whole amount" , "Every single one" .)