Policeman and soldiers discover source of oil (5)
I believe the answer is:
copra
'source of oil' is the definition.
(I've seen this before)
'policeman and soldiers' is the wordplay.
'policeman' becomes 'cop' (slang term).
'and' means one lot of letters go next to another.
'soldiers' becomes 'RA' (abbreviation for Royal Artillery).
'cop'+'ra'='COPRA'
'discover' acts as a link.
(Other definitions for copra that I've seen before include "Nutty product" , "Oil producer" , "Dried cocoa-nut kernels, source of oil" , "Coconut's kernel" , "Dried coconut-kernel" .)