Passion around contests, most substantial (8)
I believe the answer is:
heaviest
'most substantial' is the definition.
Both the definition and answer are superlative adjectives. Perhaps you can see an association between them that I don't see?
'passion around contests' is the wordplay.
'passion' becomes 'heat' ('heat' can be a synonym of 'passion').
'around' indicates putting letters inside.
'contests' becomes 'vies' (I am not sure about this - if you are sure you should believe this answer much more).
'heat' placed around 'vies' is 'HEAVIEST'.
Can you help me to learn more?
(Other definitions for heaviest that I've seen before include "Of comparatively great physical weight or density" , "Weighing the most" , "Weightiest" , "Most difficult to shift" , "Most weighty" .)