Popular team locked up (6)
I believe the answer is:
inside
'locked up' is the definition.
(I've seen this before)
'popular team' is the wordplay.
'popular' becomes 'in' (something that's 'in' is fashionable).
'team' becomes 'side' (both can mean a sports team).
'in'+'side'='INSIDE'
(Other definitions for inside that I've seen before include "Privileged" , "serving porridge" , "Enid is (anag) - within" , "being held" , "undercover" .)