Mischievous about remaining beggar (10)
I believe the answer is:
impoverish
'beggar' is the definition.
(thesaurus)
'mischievous about remaining' is the wordplay.
'mischievous' becomes 'impish' ('impish' can be a synonym of 'mischievous').
'about' indicates putting letters inside.
'remaining' becomes 'over' (as in 'we ate them all but had one over').
'impish' enclosing 'over' is 'IMPOVERISH'.
(Other definitions for impoverish that I've seen before include "beggar" , "Make poor" , "Spoil the quality of" .)