A month with English theatre's top group (6)
I believe the answer is:
septet
'group' is the definition.
The answer is an example.
'a month with english theatre's top' is the wordplay.
'a month' becomes 'sept' (Sept is an example).
'with' is a charade indicator (letters next to each other).
'top' suggests taking the first letters (in a down clue, the first letter is top).
The first letters of 'english theatres' is 'et'.
'sept'+'et'='SEPTET'
(Other definitions for septet that I've seen before include "Musical group" , "Several players" , "Group of seven voices" , "piece by Beethoven" , "A set of seven" .)