"It keeps the light out, mum," you say (7)
I believe the answer is:
shutter
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'the light out mum you say' is the wordplay.
'the light out mum you' becomes 'sh' (I can't justify this - if you can you should give a lot more credence to this answer).
'say' becomes 'utter' (to utter something is to speak or say it).
'sh'+'utter'='SHUTTER'
'keeps' is the link.
Can you help me to learn more?
(Other definitions for shutter that I've seen before include "Hinged window cover" , "Moveable window screen" , "Device on a camera, it opens and closes" , "It helps provide shade" , "Hinged window blind" .)