Plant with Jennies in the middle and at the back (6)
I believe the answer is:
astern
'at the back' is the definition.
(I've seen this before)
'plant with jennies in the middle' is the wordplay.
'plant' becomes 'aster' (aster is a kind of plant).
'with' is a charade indicator (letters next to each other).
'in the middle' indicates the central letters.
The central letter of 'jennies' is 'n'.
'aster'+'n'='ASTERN'
'and' acts as a link.
(Other definitions for astern that I've seen before include "Behind the back of a ship" , "Backward" , "Abaft" , "following behind vessel" , "Towards back of boat" .)