Stop about English being reserved (5)
I believe the answer is:
quiet
'being reserved' is the definition.
(thesaurus)
'stop about english' is the wordplay.
'stop' becomes 'quit' (synonyms).
'about' indicates putting letters inside.
'english' becomes 'E' (abbreviation).
'quit' enclosing 'e' is 'QUIET'.
(Other definitions for quiet that I've seen before include "Silence, calm" , "Making little noise" , "Calm, hushed" , "Silent" , "Without sound" .)