Behind or up-to-date? Behind, stopping behind mostly (2,7)
I believe the answer is:
in arrears
'behind' is the definition.
(behind on payments)
'up-to-date? behind stopping behind mostly' is the wordplay.
'up-to-date?' becomes 'in' ('in' can mean trendy or fashionable).
'behind' says to put letters next to each other.
'stopping behind mostly' becomes 'arrears' (I can't justify this - if you can you should believe this answer much more).
'in'+'arrears'='IN ARREARS'
'or' is the link.
(Other definitions for in arrears that I've seen before include "Owing" , "Behind with payments" , "Behindhand (with rent)" , "Position when a debt remains unpaid after its due date" , "Late with payments" .)