Eager to help out with a fiver? (4)
I believe the answer is:
avid
'eager' is the definition.
('avid' can be a synonym of 'eager')
'with a fiver?' is the wordplay.
'with' means one lot of letters go next to another.
'fiver?' becomes 'vid' (I can't explain this - if you can you should believe this answer much more).
'a'+'vid' is 'AVID'.
'to help out' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for avid that I've seen before include "Keen, enthusiastic" , "Greedy - enthusiastic" , "Keenly interested" , "Fervent" , "hungry" .)