Positive aspect in United facing Porto's first team (6)
I believe the answer is:
upside
'positive aspect' is the definition.
(I've seen this in another clue)
'united facing porto's first team' is the wordplay.
'united' becomes 'U' (abbreviation).
'facing' says to put letters next to each other.
'porto's first' becomes 'p' (1st letter of 'porto').
'team' becomes 'side' (both can mean a sports team).
'u'+'p'+'side'='UPSIDE'
'in' is the link.
(Other definitions for upside that I've seen before include "Positive aspect of a situation" , "Favourable aspect" , "Top surface of something" , "advantage" .)