Bulb working repeatedly without one (5)
I believe the answer is:
onion
'bulb' is the definition.
(plant bulb)
'working repeatedly without one' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'repeatedly' says to double the letters.
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'one' becomes 'i' (Roman numeral).
'on' doubled is 'onon'.
'onon' placed around 'i' is 'ONION'.
(Other definitions for onion that I've seen before include "Pungent vegetable" , "Bulbous vegetable" , "Aromatic flavourful versatile vegetable" , "Head (slang); vegetable" , "Eye-watering vegetable" .)