Consume natural fuel when short of power (3)
I believe the answer is:
eat
'consume' is the definition.
(to consume something is to eat it)
'natural fuel when short of power' is the wordplay.
'natural fuel' becomes 'peat'.
'when short of' suggests deleting specific letters.
'power' becomes 'p' (symbol in physics).
'peat' with 'p' removed is 'EAT'.
(Other definitions for eat that I've seen before include "Be corrosive" , "Dine in a restaurant" , "Worry" , "Take food" , "put away" .)