Tall child misbehaving with Nick Sharp (10)
I believe the answer is:
improbable
'tall' is the definition.
('improbable' can be a synonym of 'tall')
'child misbehaving with nick sharp' is the wordplay.
'child misbehaving' becomes 'imp'.
'with' is a charade indicator (letters next to each other).
'nick' becomes 'rob' (I've seen this in other clues).
'sharp' becomes 'able'.
'imp'+'rob'+'able'='IMPROBABLE'
(Other definitions for improbable that I've seen before include "Far-fetched" , "Not likely" , "Highly unlikely" .)