Mob crowed without energy (5)
I believe the answer is:
crowd
'mob' is the definition.
(I've seen this before)
'crowed without energy' is the wordplay.
'without' indicates named letters should be taken away.
'energy' becomes 'e' (physics symbol as in E=mc2).
'crowed' with 'e' removed is 'CROWD'.
(Other definitions for crowd that I've seen before include "Spectators" , "Mob of people" , "Large number of people gathered together" , "See 1" , "Large group of people" .)