Chorister beginning to develop got much bigger (7)
I believe the answer is:
trebled
'got much bigger' is the definition.
(increased threefold)
'chorister beginning to develop' is the wordplay.
'chorister' becomes 'treble' (I've seen this before).
'beginning to' says to take the initial letters.
The first letter of 'develop' is 'd'.
'treble'+'d'='TREBLED'
(Other definitions for trebled that I've seen before include "Increased threefold" , "Multiplied by three" , "produced twice as much again" .)