It means you can go ahead and practise putting in the dark (5,5)
I believe the answer is:
green light
'it means you can go ahead and practise putting in the dark' is the definition.
The definition suggests a singular noun which matches the answer.
This is the entire clue.
Can you help me to learn more?
(Other definitions for green light that I've seen before include "Go-ahead" , "What one has to go on" , "Given permission to proceed" , "Signal for go" , "something to go on" .)