An overhead light (4)
I believe the answer is:
halo
'an overhead light' is the definition.
(I've seen this in another clue)
'overhead light' is the wordplay.
I cannot really understand how this works, but
'overhead' could be 'o' (head letter of 'over') and 'o' is found within the answer.
The remaining letters 'hal' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for halo that I've seen before include "Sign of holiness" , "Circle over a saint" , "Saint's circle of light" , "Hagiological aura featured in Christian art" , "Aura representing saintliness" .)