Best friend supporting United let rip occasionally (8)
I believe the answer is:
ultimate
'best' is the definition.
(I've seen this before)
'friend supporting united let rip occasionally' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'united let' becomes 'ult' (I can't explain this - if you can you should give a lot more credence to this answer).
'occasionally' indicates alternate letters.
The alternate letters of 'rip' are 'i'.
'ult'+'i'='ulti'
'mate' put after 'ulti' is 'ULTIMATE'.
Can you help me to learn more?
(Other definitions for ultimate that I've seen before include "Furthest, final" , "Last or final in a series or process" , "The last element of a series" , "Mutilate in last possible way" , "Final or absolute" .)