Repeated denial about being cut by top of incandescent bulb (5)
I believe the answer is:
onion
'bulb' is the definition.
(plant bulb)
'repeated denial about being cut by top of incandescent' is the wordplay.
'repeated' shows that letters should be duplicated.
'denial' becomes 'no' (no is a kind of denial).
'about' says the letters should be written backwards.
'being cut by' indicates putting letters inside (some letters cut their way into a word).
'top of' says to take the initial letters.
The first letter of 'incandescent' is 'i'.
'no' reversed gives 'on'.
'on' doubled is 'onon'.
'onon' placed around 'i' is 'ONION'.
(Other definitions for onion that I've seen before include "Head (slang); vegetable" , "Alliaceous plant" , "reason for tears" , "Strong tasting vegetable" , "Edible tear-jerker" .)