Type of light bulb fitting (7)
I believe the answer is:
bayonet
'type of light bulb fitting' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bayonet that I've seen before include "Knife fixed to the end of a rifle" , "Dagger attached to 26 Across" , "Kind of knife attached to a rifle" , "one's attached to one's rifle - that's sort of fitting" , "Type of light bulb fitting" .)