Milan team contains, say, eleven? (7)
I believe the answer is:
integer
'eleven?' is the definition.
(eleven is a kind of integer)
'milan team contains say' is the wordplay.
'milan team' becomes 'Inter' (Inter Milan football team).
'contains' indicates putting letters inside.
'say' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'inter' going around 'eg' is 'INTEGER'.
(Other definitions for integer that I've seen before include "Positive or negative whole number" , "Perhaps 60,000" , "rational number remains" , "Result of counting" , "Gin tree (anag)" .)