Tropical plant, one with large swelling on root? (7)
I believe the answer is:
gumboil
'swelling on root?' is the definition.
(gum swelling over the root of a tooth)
'tropical plant one with large' is the wordplay.
'tropical plant' becomes 'gumbo' (another term for okra).
'one' becomes 'I' (Roman numeral).
'with' says to put letters next to each other.
'large' becomes 'l'.
'gumbo'+'i'+'l'='GUMBOIL'
(Other definitions for gumboil that I've seen before include "Small abscess of the mouth" , "Swelling in the mouth" .)