Concerning energy with capacity to emit (4)
I believe the answer is:
reek
'emit' is the definition.
(reeking is a kind of emiting)
'concerning energy with capacity' is the wordplay.
'concerning' becomes 're'.
'energy' becomes 'e' (physics symbol as in E=mc2).
'with capacity' becomes 'k' (I can't justify this - if you can you should believe this answer much more).
're'+'e'+'k'='REEK'
'to' is the link.
Can you help me to learn more?
(Other definitions for reek that I've seen before include "'Stink, stench (4)'" , "Smell offensively" , "Smell badly" , "Smell strongly" , "Smell bad" .)