Wall map marker (4)
I believe the answer is:
tack
'wall map marker' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tack that I've seen before include "Alter course - riding gear" , "Food; nail" , "Staple - food" , "Small nail; sail (boat) against the wind" , "Type of nail" .)