Outsiders in blue teams to boot (7)
I believe the answer is:
besides
'to boot' is the definition.
(I've seen this before)
'outsiders in blue teams' is the wordplay.
'outsiders in' suggests removing the centre.
'teams' becomes 'sides' (eg football sides).
'blue' with its centre taken out is 'be'.
'be'+'sides'='BESIDES'
(Other definitions for besides that I've seen before include "In addition, as well" , "Anyway" , "See bids (anag)" , "Apart from" , "What's more" .)