Bulb lit, current not off (5)
I believe the answer is:
onion
'bulb' is the definition.
(plant bulb)
'lit current not off' is the wordplay.
'lit' becomes 'on' (eg a light that is switched on).
'current' becomes 'I' (symbol for electric current).
'not off' becomes 'on' ('on' is the opposite to 'off').
'on'+'i'+'on'='ONION'
(Other definitions for onion that I've seen before include "Tear-inducing edible bulb" , "Aromatic flavourful versatile vegetable" , "One may be set" , "Alliaceous plant" , "Pungent edible bulb of the lily family" .)