Priest managed to conserve energy and rest (7)
I believe the answer is:
respite
'rest' is the definition.
(I know this)
'priest managed to conserve energy' is the wordplay.
'managed' indicates an anagram.
'to conserve' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'priest' anagrammed gives 'rspite'.
'rspite' enclosing 'e' is 'RESPITE'.
'and' is the link.
(Other definitions for respite that I've seen before include "A lull" , "Relief; breather" , "Period of relief (from unpleasant thing)" , "Period of temporary relief" , "interruption" .)