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' means to remove the middle letters.
'teams' becomes 'sides' (eg football sides).
'blue' with its middle removed 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" .)