Observe and note unknown energy (3)
I believe the answer is:
eye
'observe' is the definition.
(eye can mean to observe or see)
'note unknown energy' is the wordplay.
'note' becomes 'e' (musical note).
'unknown' becomes 'y' (algebra).
'energy' becomes 'e' (physics symbol as in E=mc2).
'e'+'y'+'e'='EYE'
'and' is the link.
(Other definitions for eye that I've seen before include "Globular organ" , "Organ of vision" , "Small loop or hole" , "Centre of hurricane" , "Watch closely" .)