Fish giving old men inner energy (5)
I believe the answer is:
popes
'fish' is the definition.
'old men inner energy' is the wordplay.
'old men' becomes 'pops'.
'inner' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'pops' enclosing 'e' is 'POPES'.
'giving' acts as a link.
(Other definitions for popes that I've seen before include "Pontiffs" , "Heads of the Roman Catholic church" .)