Insect left, having eaten last bit of that plant (5)
I believe the answer is:
betel
'that plant' is the definition.
(betel is a kind of plant)
'insect left having eaten last bit' is the wordplay.
'insect' becomes 'bee' (bee is a kind of insect).
'left' becomes 'l' (common abbreviation).
'having eaten' means one lot of letters goes inside another.
'last' indicates one should take the final letters.
The final letter of 'bit' is 't'.
'bee'+'l'='beel'
'beel' placed around 't' is 'BETEL'.
'of' is the link.
(Other definitions for betel that I've seen before include "stimulatingly chewed in Asia" , "Asian tree, whose nuts and leaves are chewed as a narcotic" , "Leaf or nut chewed in SE Asia" , "leaves to chew" , "Asian feather palm" .)