Trap northern elk heading off (5)
I believe the answer is:
noose
'trap' is the definition.
(I know that noose is a type of trap)
'northern elk heading off' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'elk' becomes 'moose' (synonyms).
'heading off' means to remove the first letter.
'moose' with its initial letter taken away is 'oose'.
'n'+'oose'='NOOSE'
(Other definitions for noose that I've seen before include "Catcher" , "Hangman's equipment" , "This exemplifies marriage for some" , "Loop with sliding knot in a rope" , "Loop in rope that tightens when pulled" .)