Tiller used to create border around fourth field (4)
I believe the answer is:
helm
'tiller' is the definition.
(I've seen this in another clue)
'border around fourth field' is the wordplay.
'border' becomes 'hem' (I've seen this before).
'around' indicates putting letters inside.
'fourth field' becomes 'l' (4th letter of 'field').
'hem' going around 'l' is 'HELM'.
'used to create' is the link.
(Other definitions for helm that I've seen before include "Steer (a boat or ship)" , "Tiller, steering wheel" , "Position of steerer" , "Steering gear" , "Wheel that controls ship's rudder" .)