Persistent about fast time? Not so much (10)
I believe the answer is:
relentless
'persistent' is the definition.
(I know that persistent can be written as relentless)
'about fast time? not so much' is the wordplay.
'about' becomes 're' (regarding).
'fast time?' becomes 'lent' (Lent is a time for fasting).
'not so much' becomes 'less' (I've seen this in another clue).
're'+'lent'+'less'='RELENTLESS'
(Other definitions for relentless that I've seen before include "sustained" , "Harsh, inflexible" , "Unyielding - implacable" , "Uncompromising" , "Inexorable" .)