Safeguard Poles in English river (6)
I believe the answer is:
ensure
'safeguard' is the definition.
(thesaurus)
'poles in english river' is the wordplay.
'poles' becomes 'ns' (North and South poles).
'in' is an insertion indicator.
'english' becomes 'E' (abbreviation).
'river' becomes 'ure' (river in Yorkshire).
'e'+'ure'='eure'
'ns' going inside 'eure' is 'ENSURE'.
(Other definitions for ensure that I've seen before include "Make certain, confirm" , "Make certain for the future" , "See run (anag)" , "Make certain that (something occurs)" , "Guarantee" .)