Idle fool on line (4)
I believe the answer is:
loaf
'idle' is the definition.
(loafing is a kind of idling)
'fool on line' is the wordplay.
'fool' becomes 'oaf' (I've seen this before**).
'on' means one lot of letters go next to another.
'line' becomes 'l'.
'oaf' put after 'l' is 'LOAF'.
(Other definitions for loaf that I've seen before include "Piece of bread" , "Baguette or cob, perhaps" , "more than just crumbs!" , "Lounge about" , "Pass time idly" .)