Be first in the field at Land's End (4)
I believe the answer is:
lead
'be first' is the definition.
(lead can mean leading or primary)
'field at land's end' is the wordplay.
'field' becomes 'lea' (lea can mean a meadow or field of grass).
'at' is a charade indicator (letters next to each other).
'end' suggests the final letters.
The final letter of 'land' is 'd'.
'lea'+'d'='LEAD'
'in the' is the link.
(Other definitions for lead that I've seen before include "Soft, heavy metal" , "initiative" , "chain" , "Church windows include this" , "Guide by going in front" .)