First of nurserymen supporting plant at the rear (6)
I believe the answer is:
astern
'at the rear' is the definition.
(I've seen this before)
'first of nurserymen supporting plant' is the wordplay.
'first of nurserymen' becomes 'n' (1st letter of 'nurserymen').
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'plant' becomes 'aster' (aster is a kind of plant).
'n' after 'aster' is 'ASTERN'.
(Other definitions for astern that I've seen before include "The wake is" , "at the vessel's rear" , "Aft" , "back for sailors" , "Towards a ship's rear" .)