Popular fellow in it has energy, boundless (8)
I believe the answer is:
infinite
'boundless' is the definition.
(similar in meaning)
'popular fellow in it has energy' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'has' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mc2).
'in'+'f'+'in'+'it'+'e'='INFINITE'
(Other definitions for infinite that I've seen before include "Never-ending" , "Without boundaries" , "Having no boundaries or limits" , "incredibly large" , "Endless" .)