Unfortunately, lie: and I'd lose this source of energy (6,3)
I believe the answer is:
diesel oil
'source of energy' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'unfortunately lie and i'd lose' is the wordplay.
'unfortunately' indicates anagramming the letters (a bad or unfortunate spelling).
'and' means one lot of letters go next to another.
'id'+'lose'='idlose'
'lie' put after 'idlose' is 'idloselie'.
'idloselie' is an anagram of 'DIESEL OIL'.
'this' is the link.
Can you help me to learn more?
(Another definition for diesel oil that I've seen is " Type of fuel".)