Wandering after the devil is getting better (9)
I believe the answer is:
improving
'is getting better' is the definition.
(I've seen this before)
'wandering after the devil' is the wordplay.
'wandering' becomes 'roving' (synonyms).
'after' is a charade indicator (letters next to each other).
'the devil' becomes 'imp' (devilish creature).
'roving' put after 'imp' is 'IMPROVING'.
(Other definitions for improving that I've seen before include "Making better" , "Getting better" , "Recovering" .)