Cork a red light! (7)
I believe the answer is:
stopper
'cork a red light' is the definition.
'stopper' can be an answer for 'cork' (a cork stops a bottle). I am unsure of the 'a red light' bit.
'cork a' is the wordplay.
'cork' becomes 'stop' (I've seen this in another clue).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'stop'+'per'='STOPPER'
Can you help me to learn more?
(Other definitions for stopper that I've seen before include "One blocks" , "Plug for a bottle" , "Plug - cork" , "One who checks" , "Maybe cork" .)