Recycling unlimited energy is environmentally friendly (5)
I believe the answer is:
green
'environmentally friendly' is the definition.
(I've seen this before)
'recycling unlimited energy' is the wordplay.
'recycling' says the letters should be written backwards (the letters cycle back).
'unlimited' becomes 'ne' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'energy' becomes 'erg'.
'ne'+'erg'='neerg'
'neerg' reversed gives 'GREEN'.
'is' is the link.
Can you help me to learn more?
(Other definitions for green that I've seen before include "bowlers here?" , "Money" , "one may be putting on" , "Fresh - environmentally friendly - colour" , "Environmentally concerned" .)