Beneath border one has salad plant (6)
I believe the answer is:
endive
'salad plant' is the definition.
(I've seen this before)
'beneath border one has' is the wordplay.
'beneath' means one lot of letters go next to another (in a down clue, some letters go below others).
'border' becomes 'end' (end can mean a border or edge).
'one has' becomes 'I've' ('one has' can formally or humorously mean 'I have').
'end' next to 'ive' is 'ENDIVE'.
(Other definitions for endive that I've seen before include "Portion of salad" , "salad ingredient" , "Veined (anag) - salad plant" , "Type of chicory" , "Edible plant" .)