Farm workers start to trade in dye (7)
I believe the answer is:
pigment
'dye' is the definition.
(I've seen this before)
'farm workers start to trade' is the wordplay.
'farm workers' becomes 'pigmen' (pigman can mean a man who looks after pigs).
'start to' says to take the initial letters.
The initial letter of 'trade' is 't'.
'pigmen'+'t'='PIGMENT'
'in' is the link.
(Other definitions for pigment that I've seen before include "Colour or dye" , "become imbued" , "Colouring matter" , "Paint" , "Colouring substance" .)