Formality from McEnroe sadly variable (8)
I believe the answer is:
ceremony
'formality' is the definition.
(ceremony is a kind of formality)
'mcenroe sadly variable' is the wordplay.
'sadly' indicates anagramming the letters.
'variable' becomes 'y' (algebra).
'mcenroe' is an anagram of 'ceremon'.
'ceremon'+'y'='CEREMONY'
'from' acts as a link.
(Other definitions for ceremony that I've seen before include "Pomp, formality" , "Event of ritual significance" , "Rite or formal act for special occasion" , "Kind of ritual - don't stand on it" , "Ritual celebration" .)