Unexpectedly gaining all oil (5-5)
I believe the answer is:
ilang-ilang
'oil' is the definition.
(ilang-ilang is a kind of oil)
'unexpectedly gaining all' is the wordplay.
'unexpectedly' indicates anagramming the letters.
'gaining'+'all'='gainingall'
'gainingall' with letters rearranged gives 'ILANG-ILANG'.
I've seen this clue in The Independent.
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