Head off evidence of bovine activity in most of the canal trail (7)
I believe the answer is:
towpath
'canal trail' is the definition.
(path alongside a canal)
'head off evidence of bovine activity in most of the' is the wordplay.
'head off' means to remove the first letter.
'evidence of bovine activity' becomes 'cowpat' (cows are bovine animals).
'in' indicates putting letters inside.
'most of' means to remove the last letter.
'cowpat' with its initial letter taken off is 'owpat'.
'the' with its last letter taken away is 'th'.
'owpat' going inside 'th' is 'TOWPATH'.
(Other definitions for towpath that I've seen before include "Canalside track" , "Canalside way" , "Canal walk" , "Track beside a canal" , "Track once used to pull barges on canals" .)