Tempestuous tale entertaining millions (6)
I believe the answer is:
stormy
'tempestuous' is the definition.
(I know that tempestuous can be written as stormy)
'tale entertaining millions' is the wordplay.
'tale' becomes 'story' (synonyms).
'entertaining' means one lot of letters goes inside another (inviting in, as for dinner).
'millions' becomes 'm' (abbreviation).
'story' going around 'm' is 'STORMY'.
(Other definitions for stormy that I've seen before include "'. . . . . . Weather' (sung by Billie Holiday, say)" , "Emotional" , "Turbulent and tempestuous" , "Passionate" , "Rough" .)