Popular team, fundamentally (6)
I believe the answer is:
inside
'fundamentally' is the definition.
(thesaurus)
'popular team' is the wordplay.
'popular' becomes 'in'.
'team' becomes 'side' (I've seen this before**).
'in'+'side'='INSIDE'
(Other definitions for inside that I've seen before include "locked up" , "A sort of job" , "(Of information) from a confidential source" , "Internal" , "Behind bars (sl.)" .)