Become active in English sport, joining gym (5)
I believe the answer is:
erupt
'become active' is the definition.
(eg a volcano erupting)
'english sport joining gym' is the wordplay.
'english' becomes 'e' (abbreviation).
'sport' becomes 'RU' (rugby union).
'joining' is a charade indicator (letters next to each other).
'gym' becomes 'PT' (Physical Training).
'e'+'ru'+'pt'='ERUPT'
'in' acts as a link.
(Other definitions for erupt that I've seen before include "Blow up like volcano" , "Discharge" , "Break out like Etna" , "Break forth" , "Explode" .)