Football team admitting former champion's top is cool (9)
I believe the answer is:
unexcited
'cool' is the definition.
(similar in meaning)
'football team admitting former champion's top' is the wordplay.
'football team' becomes 'united' (eg Manchester United).
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'former' becomes 'ex' (prefix meaning former or onetime).
'top' says to take the initial letters (in a down clue, the first letter is top).
The first letter of 'champions' is 'c'.
'ex'+'c'='exc'
'united' enclosing 'exc' is 'UNEXCITED'.
'is' is the link.
(Other definitions for unexcited that I've seen before include "Impassive" , "In need of stimulation" , "Showing little emotion" , "Not emotionally roused" .)