Arrest in motion — time to begin again (7)
I believe the answer is:
restart
'begin again' is the definition.
('start' can be a synonym for 'begin' so perhaps 'restart' is OK for 'begin again')
'arrest in motion time' is the wordplay.
'in motion' indicates an anagram (I've seen this in other clues).
'time' becomes 't' (abbreviation).
'arrest' is an anagram of 'restar'.
'restar'+'t'='RESTART'
'to' acts as a link.
(Other definitions for restart that I've seen before include "Jumping the gun will mean" , "Begin anew" , "begin afresh" , "launch again" , "Resume" .)