Foolish Sally lifted up jumper (8)
I believe the answer is:
crackpot
'foolish' is the definition.
(I've seen this before)
'sally lifted up jumper' is the wordplay.
'sally' becomes 'crack' ('crack' can be a synonym of 'sally').
'lifted up' says the letters should be written in reverse.
'jumper' becomes 'top'.
'top' reversed gives 'pot'.
'crack'+'pot'='CRACKPOT'
(Other definitions for crackpot that I've seen before include "Crank" , "Foolish" , "Idiot" , "Eccentric (colloq.)" , "Bats" .)