Model not a sensible person? No need for that! (6,2)
I believe the answer is:
forget it
'no need for that' is the definition.
(phrase meaning 'don't worry about that')
'model not a sensible person?' is the wordplay.
'model' becomes 'forge' (forge can mean to shape or model).
'not a sensible person?' becomes 'tit' (informal term for a stupid person).
'forge'+'tit'='FORGET IT'
(Other definitions for forget it that I've seen before include "Not a chance" , "Think no more of the matter" , "ignore something" , "Let it go!" , "There's no need for apologies" .)