Clear up problem for decent amount (4,3)
I believe the answer is:
tidy sum
'decent amount' is the definition.
(a decent amount of money)
'clear up problem' is the wordplay.
'clear up' becomes 'tidy' (I've seen this before).
'problem' becomes 'sum' (as in a mathematical problem).
'tidy'+'sum'='TIDY SUM'
'for' acts as a link.
(Other definitions for tidy sum that I've seen before include "A lot of money" , "Considerable amount" .)