Disused old tractor starts for Dorothy (3)
I believe the answer is:
dot
'dorothy' is the definition.
(nickname for someone called Dorothy)
'disused old tractor starts' is the wordplay.
'starts' says to take the initial letters.
The initial letters of 'disused old tractor' is 'DOT'.
'for' is the link.
(Other definitions for dot that I've seen before include "Little mark" , "scatter" , "Speck or period" , "Spot, speck" , "Small round spot" .)