Insect left, having eaten last bit of that plant (5)
I believe the answer is:
Here is my best explanation:
'that plant' is the definition.
(betel is a kind of plant)
'insect left having eaten last bit' is the subsidiary indication.
'insect' becomes 'bee' (bee is a kind of insect).
'left' becomes 'l' (common abbreviation).
'having eaten' indicates putting letters inside.
'last' suggests the final letters.
The last letter of 'bit' is 't'.
'beel' going 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", "Plant leaf chewed in the East", "Something stimulating", "Asian palm and nut", "Palm with nuts for chewing" and "Palm with leaf and nuts chewed".)