Pure speed -- beyond maximum speed! (6)
I believe the answer is:
chaste
'pure' is the definition.
(chaste means abstaining from sex)
'speed beyond maximum speed' is the wordplay.
'speed' becomes 'haste' (haste is a kind of speed).
'beyond' is a charade indicator (letters next to each other).
'maximum speed' becomes 'c' (I've seen this in other clues).
'haste' after 'c' is 'CHASTE'.
(Other definitions for chaste that I've seen before include "Showing virtue of kind" , "Pure and virginal" , "avoiding relations" , "Decorous" , "'Celibate, virginal (6)'" .)