Advanced by one large plant (6)
I believe the answer is:
lentil
'plant' is the definition.
(lentil plant is a kind of plant)
'advanced by one large' is the wordplay.
'advanced' becomes 'lent' (money).
'by' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'large' becomes 'l'.
'lent'+'i'+'l'='LENTIL'
(Other definitions for lentil that I've seen before include "Edible dried pulse" , "Type of soup" , "Lintel (anag.)" , "Small edible seed" , "Pulse for soup etc" .)