Energy shown by King enveloped in obvious strain? (9)
I believe the answer is:
overexert
'strain?' is the definition.
(I've seen this before)
'energy shown by king enveloped in obvious' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'shown by' says to put letters next to each other.
'king' becomes 'rex' ('king' in Latin).
'enveloped in' indicates putting letters inside.
'obvious' becomes 'overt' (similar in meaning).
'e' put after 'rex' is 'rexe'.
'rexe' inserted within 'overt' is 'OVEREXERT'.
(Other definitions for overexert that I've seen before include "use too much" , "Make too much effort" , "exercise unduly?" , "work too hard" , "strain" .)