Old magistrate always going round full of energy (5)
I believe the answer is:
reeve
'old magistrate' is the definition.
(I've seen this before)
'always going round full of energy' is the wordplay.
'always' becomes 'ever' (synonyms).
'going round' says the letters should be written in reverse.
'full of' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'ever' backwards is 'reve'.
'reve' placed around 'e' is 'REEVE'.
(Other definitions for reeve that I've seen before include "Ancient magistrate" , "Historical magistrate" , "Old chief magistrate" , "Feudal official of medieval England" , "Thread (rope) through a ring, old official" .)