Philanthropist abandons half the Hebridean islands (4)
I believe the answer is:
uist
'hebridean islands' is the definition.
(Uist is a group of Hebridean islands)
'philanthropist abandons half' is the wordplay.
'philanthropist' becomes 'altruist' (synonyms).
'abandons half' means to take half the letters.
'altruist' halved is 'UIST'.
'the' acts as a link.
(Other definitions for uist that I've seen before include "Hebridean islands, North and South ____" , "North and South islands of the Outer Hebrides" , "Hebridean island" .)