Island about to enter into obligation (5)
I believe the answer is:
tiree
'island' is the definition.
(Scottish island)
'about to enter into obligation' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to enter into' indicates putting letters inside (I've seen this in other clues).
'obligation' becomes 'tie' (I've seen this before).
're' going within 'tie' is 'TIREE'.
(Other definitions for tiree that I've seen before include "One of the Inner Hebrides" , "Hebridean island" , "Scottish island of the Inner Hebrides" , "Isle of Inner Hebrides" , "small island" .)