Popular business with periodically immense earnings (7)
I believe the answer is:
incomes
'earnings' is the definition.
(income can mean earnings or money)
'popular business with periodically immense' is the wordplay.
'popular' becomes 'in' (something that's 'in' is fashionable).
'business' becomes 'co' (short for company).
'with' is a charade indicator (letters next to each other).
'periodically' means one should take alternating letters (letters must be selected 'every so often').
The alternate letters of 'immense' are 'mes'.
'in'+'co'+'mes'='INCOMES'
(Another definition for incomes that I've seen is " Revenues, salaries".)