Underplay but note one's energy (8)
I believe the answer is:
minimise
'underplay' is the definition.
('underplay'->'minimize' is in my internal thesaurus and 'ize'->'ise')
'note one's energy' is the wordplay.
'note' becomes 'minim' (minim is a kind of note**).
'one's' becomes 'is' (roman numeral).
'energy' becomes 'e' (physics symbol as in E=mci).
'minim'+'is'+'e'='MINIMISE'
'but' acts as a link.
(Other definitions for minimise that I've seen before include "Diminish" , "Scale down" , "Shrink" , "Downplay, understate" , "Make as small as possible" .)