Back in a tick (6)
I believe the answer is:
second
This clue is a double definition.
'back' is the first definition.
('second' can be a synonym of 'back')
'a tick' is the second definition.
(thesaurus)
'in' is the link.
(Other definitions for second that I've seen before include "Unsurpassed" , "Short-lived" , "Moment in time" , "Attendant in a duel or boxing match" , "Next to first" .)