Forebodings I should have mentioned, harbouring ill-feeling about island (13)

'forebodings' is the definition.
('presentiment' can be a synonym of 'foreboding')

'feeling about island' is the wordplay.
'feeling' becomes 'sentiment' (a sentiment is a feeling or view).
'about' means one lot of letters goes inside another.
'island' becomes 'pres' (I can't justify this - if you can you should believe this answer much more).
'sentiment' placed into 'pres' is 'PRESENTIMENTS'.

'i should have mentioned harbouring ill' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.

Can you help me to learn more?