Informal jumper: knitter’s first piece (4)
I believe the answer is:
rook
'piece' is the definition.
(rook is a kind of piece)
'informal jumper knitter's first' is the wordplay.
'informal jumper' becomes 'roo' (informal name for a kangaroo).
'first' says to take the initial letters.
The first letter of 'knitter' is 'k'.
'roo'+'k'='ROOK'
(Other definitions for rook that I've seen before include "Man in game" , "Chess piece or crow" , "man in marginalised position, initially" , "journalist Jean" , "Gregarious crow" .)