A trail going north for police officer (5)
I believe the answer is:
garda
'police officer' is the definition.
(member of the Irish police)
'a trail going north' is the wordplay.
'trail' becomes 'drag' (synonyms).
'going north' says the letters should be written in reverse.
'a'+'drag'='adrag'
'adrag' reversed gives 'GARDA'.
'for' acts as a link.
(Other definitions for garda that I've seen before include "Policeman of the Irish Republic" , "An Irish limb of the law" , "Italian lake - Irish policeman" , "Irish police officer" , "Cop" .)