It can help if you turn up the radiator a bit (3)
I believe the answer is:
aid
'it can help if you turn' is the definition.
The definition suggests a singular noun which matches the answer.
'up the radiator a bit' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'a bit' indicates a hidden word (the answer is formed by a bit of the clue).
'dia' can be found hidden inside 'radiator'.
'dia' back-to-front is 'AID'.
Can you help me to learn more?
(Other definitions for aid that I've seen before include "encouragement" , "A hand" , "welfare" , "Help or succour" , "Help. Assist" .)