WI song so neglected in Brussels it upset oil suppliers (9)
I believe the answer is:
Here is my best explanation:
'oil suppliers' is the definition.
(sources of eucalyptus oil)
'wi song so neglected in brussels it upset' is the wordplay.
'wi song' becomes 'calypso' (type of song from the West Indies).
'neglected' indicates named letters should be taken away (neglect can mean to ignore).
'in' means one lot of letters goes inside another.
'brussels' becomes 'eu' (Brussels is a metonym for the European Union).
'upset' says the letters should be written backwards.
'calypso' with 'so' taken away is 'calyp'.
'it' in reverse letter order is 'ti'.
'calyp' placed into 'euti' is 'EUCALYPTI'.
(Other definitions for eucalypti that I've seen before include "Oil producers", "shrubs", "Gums", "Trees", "Antipodean natives", "oil producers", "Gum trees", "Oil sources" and "Australian trees".)