... man in jumper and Knights top (4)
I believe the answer is:
rook
'man' is the definition.
(rook is a kind of man)
'jumper and knights top' is the wordplay.
'jumper' becomes 'roo' (short for kangaroo, something which jumps).
'and' means one lot of letters go next to another.
'top' says to take the initial letters.
The first letter of 'knights' is 'k'.
'roo'+'k'='ROOK'
'in' is the link.
(Other definitions for rook that I've seen before include "Castle -- bird" , "Defraud; bird; chessman" , "Bird; playing piece" , "man for moving" , "Steal - castle" .)