It won't throw much light on the subject (5)
I believe the answer is:
gleam
'light on the subject' is the definition.
'gleam' can be an answer for 'light' (I have seen 'brief light' mean 'gleam' so perhaps 'light' could also mean 'gleam'). I am not certain of the 'on the subject' bit.
'it won't throw much' is the wordplay.
I cannot quite see how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is located in the answer.
The remaining letters 'glam' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for gleam that I've seen before include "Glimmer, lustre" , "Brief show of soft light" , "Sparkle, glimmer" , "Glisten" , "Flash of light" .)