Vulgar fellow full of energy (5)
I believe the answer is:
cheap
'vulgar' is the definition.
(synonyms)
'fellow full of energy' is the wordplay.
'fellow' becomes 'chap' (both can mean a man).
'full of' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mch).
'chap' going around 'e' is 'CHEAP'.
(Other definitions for cheap that I've seen before include "Inexpensive or tastelessy showy" , "Base" , "not doing much damage" , "Not expensive - trashy" , "Budget -cheesy" .)