Company prospector’s casing a source of oil (5)
I believe the answer is:
copra
'source of oil' is the definition.
(I've seen this before)
'company prospector's casing a' is the wordplay.
'company' becomes 'co' (abbreviation).
'casing' suggests removing the centre (the outside letters are the case of the word).
'prospector' with its centre removed is 'pr'.
'co'+'pr'+'a'='COPRA'
(Other definitions for copra that I've seen before include "Shell offering oil" , "Nutty product" , "Coconuts provide" , "bit of a nut" , "Dried coconut meat" .)