On a second time, second time! (7)
I believe the answer is:
against
'on' is the definition.
(against can mean on or adjacent to)
'a second time second time' is the wordplay.
'a second time' becomes 'again' (again means for a second time).
'second' becomes 's' (common abbreviation).
'time' becomes 't' (abbreviation).
'again'+'s'+'t'='AGAINST'
(Other definitions for against that I've seen before include "Versus - in contact with" , "In competition with" , "Opposing" , "Abutting" , "on the other side" .)