Convenient amount can be a problem (7)
I believe the answer is:
handful
'convenient amount' is the definition.
'handful' can be an answer for 'amount' (handful is a kind of amount). I'm unsure of the 'convenient' bit.
'a problem' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'can be' is the link.
Can you help me to learn more?
(Other definitions for handful that I've seen before include "Someone hard to control" , "'Small number, a fistful (7)'" , "Small number - difficult person or thing" , "Something hard to control - small number" , "Perhaps lively child" .)