Most demanding fly half following matches on replay (8)
I believe the answer is:
stiffest
'most demanding' is the definition.
(stiff can mean severe or harsh)
'fly half following matches on replay' is the wordplay.
'fly half' becomes 'fest' (I am not sure about this - if you are sure you should believe this answer much more).
'following' says to put letters next to each other.
'matches' becomes 'fits' ('fit' can be a synonym of 'match').
'on replay' says the letters should be written in reverse (as in rewinding something).
'fits' backwards is 'stif'.
'fest' put after 'stif' is 'STIFFEST'.
(Other definitions for stiffest that I've seen before include "Most rigid or starched" , "Most difficult" .)