Laugh at new key farm worker (4)
I believe the answer is:
hand
'farm worker' is the definition.
(I've seen this in another clue)
'laugh at new key' is the wordplay.
'laugh' becomes 'ha'.
'at' means one lot of letters go next to another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'key' becomes 'd' (musical key).
'ha'+'n'+'d'='HAND'
(Other definitions for hand that I've seen before include "Give worker" , "Worker - penmanship - four inches" , "Unit of measurement of a horse's height" , "writing" , "Player's cards" .)