Round island, left healthy source of fuel (7)
I believe the answer is:
oilwell
'source of fuel' is the definition.
'oilwell' can be an answer for 'source' (I have seen 'source of riches' mean 'oilwell' so perhaps 'source' could also mean 'oilwell'). I'm not sure about the 'of fuel' bit.
'round island left healthy' is the wordplay.
'round' becomes 'o' (o is a circular letter).
'island left' becomes 'il' (this might be a standard abbreviation of which I'm unaware).
'healthy' becomes 'well' (similar in meaning).
'o'+'il'+'well'='OILWELL'
Can you help me to learn more?
(Another definition for oilwell that I've seen is " source of riches".)