Drone to exist on energy (3)
I believe the answer is:
bee
'drone' is the definition.
(I've seen this in another clue)
'exist on energy' is the wordplay.
'exist' becomes 'be' (synonyms).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'energy' becomes 'e' (physics symbol as in E=mce).
'be'+'e'='BEE'
'to' acts as a link.
(Other definitions for bee that I've seen before include "A honey-maker" , "Busy person" , "Insect, may be in bonnet" , "Insect that bumbles?" , "Hive worker" .)