Monetary penalty that's perfectly acceptable (4)
I believe the answer is:
fine
This clue is a double definition.
'monetary penalty' is the first definition.
(I've seen this in another clue)
'perfectly acceptable' is the second definition.
(fine can mean satisfactory)
'that's' acts as a link.
(Other definitions for fine that I've seen before include "Excellent kind of monetary penalty" , "Satisfactory" , "Acceptable - punishment" , "A financial penalty - excellent" , "Super" .)