Stadium’s healthy upsurge in crowd feeling (11)
I believe the answer is:
groundswell
'upsurge in crowd feeling' is the definition.
(a groundswell is an increase in popular opinion)
'stadium's healthy' is the wordplay.
'stadium's' becomes 'ground's' (eg a football ground).
'healthy' becomes 'we'll' (similar in meaning).
'grounds'+'well'='GROUNDSWELL'
(Other definitions for groundswell that I've seen before include "Forming opinion" , "Shift in public opinion" , "Surge of public opinion" , "Build-up of public opinion" , "Movement of public opinion" .)