Marker Irish are able to put outside (5)
I believe the answer is:
cairn
'marker' is the definition.
(I've seen this before)
'irish are able to put outside' is the wordplay.
'irish' becomes 'ir' (abbreviation).
'are able' becomes 'can'.
'to put outside' is an insertion indicator.
'ir' inserted inside 'can' is 'CAIRN'.
(Other definitions for cairn that I've seen before include "Stone-pile landmark" , "dog" , "Mound of stones on mountain top" , "Burial mound" , "Small terrier" .)