Stop odd fellow, full of energy (5)
I believe the answer is:
cease
'stop' is the definition.
(I know that stop can be written as cease)
'odd fellow full of energy' is the wordplay.
'odd fellow' becomes 'case' (old-fashioned informal term for a strange person).
'full of' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'case' going around 'e' is 'CEASE'.
(Other definitions for cease that I've seen before include "Culminate" , "Stop, terminate" , "Desist" , "Come to an end" , "Stop, give over" .)