Organised whole of largest alto section (7)
I believe the answer is:
gestalt
'organised whole' is the definition.
(psychological term for a whole concept)
'of largest alto section' is the wordplay.
'of' indicates the answer is hidden within the clue.
'GESTALT' is hidden within 'largest alto section'.
(Other definitions for gestalt that I've seen before include "Organised whole, more than the sum of its parts" , "establish a pattern" , "whole of psychology" , "Pattern perceived as whole" .)