Trail takes opposite directions back (7)
I believe the answer is:
sponsor
'back' is the definition.
(thesaurus)
'trail takes opposite directions' is the wordplay.
'trail' becomes 'spoor' (spoor is a kind of trail).
'takes' means one lot of letters goes inside another.
'opposite directions' becomes 'ns' (opposite compass points).
'spoor' placed around 'ns' is 'SPONSOR'.
(Other definitions for sponsor that I've seen before include "One financially supporting event" , "One pledging money" , "Patron or godparent" , "Provide finance for" , "One funding an event" .)