Best time to register (6)
I believe the answer is:
record
I believe this clue is a triple definition!
'best' is the first definition.
('record' can be similar in meaning to 'best')
'time' is the second definition.
(thesaurus)
'to' acts as a link.
'register' is the third definition.
('record' can be a synonym of 'register')
(Other definitions for record that I've seen before include "Log - so far unsurpassed" , "It may get broken" , "Best or worst performance" , "Set down in permanent form" , "Written report - best so far!" .)