Poles breaking up trail back (7)
I believe the answer is:
sponsor
'back' is the definition.
(I've seen this before)
'poles breaking up trail' is the wordplay.
'poles' becomes 'NS' (North and South poles).
'breaking up' means one lot of letters goes inside another (inserted letters slice into or 'break up' the word).
'trail' becomes 'spoor' (spoor is a kind of trail).
'ns' placed within 'spoor' is 'SPONSOR'.
(Other definitions for sponsor that I've seen before include "Back financially" , "One paying to support" , "One giving money in support of an event" , "One pledging money" , "One financially supporting an activity" .)