Foul where old team breaks common sense (7)
I believe the answer is:
noxious
'foul' is the definition.
(I've seen this before)
'old team breaks common sense' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'team' becomes 'XI' (a football team, from Roman numerals for 11).
'breaks' is an insertion indicator (inserted letters break into the word).
'common sense' becomes 'nous' (nous is a kind of common sense).
'o'+'xi'='oxi'
'oxi' going within 'nous' is 'NOXIOUS'.
'where' acts as a link.
(Other definitions for noxious that I've seen before include "Harmful to health" , "Disgusting" , "Unwholesome" , "foul" , "bad for you" .)