Bird heading off reaches Scottish island (4)
I believe the answer is:
inch
'scottish island' is the definition.
(I've seen this before)
'bird heading off' is the wordplay.
'bird' becomes 'finch' (finch is a kind of bird).
'heading off' means to remove the first letter.
'finch' with its first letter taken off is 'INCH'.
'reaches' is the link.
(Other definitions for inch that I've seen before include "Move in tiny increments" , "2.54 centimetres" , "Imperial unit of length" , "Subdivision of 14 down" , "Move very slowly - a very short distance" .)