Watching rival side (7)
I believe the answer is:
viewing
'watching' is the definition.
(viewing is a kind of watching)
'rival side' is the wordplay.
'rival' becomes 'vie' (I've seen this in another clue).
'side' becomes 'wing' (as in left and right wings).
'vie'+'wing'='VIEWING'
(Other definitions for viewing that I've seen before include "Opportunity to see" , "people in front of TV?" , "Surveying" , "What's done in front of the TV" .)