Quiet when the English lesson is in progress outside (9)
I believe the answer is:
noiseless
'quiet' is the definition.
(similar in meaning)
'the english lesson is in progress outside' is the wordplay.
I cannot quite see how this works, but
'english' could be 'e' (abbreviation as in OED) and 'e' is found in the answer.
'is' is present in the answer.
an anagram of 'lesson' is 'noless' which is located in the left over letters.
No letters remain.
This explanation may well be incorrect...
'when' is the link.
Can you help me to learn more?
(Other definitions for noiseless that I've seen before include "Lionesses (anag) - making no sound" , "Mum" , "with a silencer?" , "unsound?" , "Silent, quiet" .)