Many have to admit it's a plant (6)
I believe the answer is:
mallow
'a plant' is the definition.
(mallow matches up with the definition a plant)
'many have to admit' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'have' is a charade indicator (letters next to each other).
'to admit' becomes 'allow' (allowing is a kind of admiting).
'm'+'allow'='MALLOW'
'it's' is the link.
(Other definitions for mallow that I've seen before include "Downy-leaved plant and Cork town" , "'Cork town, a plant and a sweet (6)'" , "Shrub" , "Hairy-stemmed plant" .)