A step taken by theme's essential character to become a film star (7)
I believe the answer is:
astaire
'a film star' is the definition.
(Fred Astaire)
'a step taken by theme's essential character' is the wordplay.
'step' becomes 'stair' (synonyms).
'taken by' is a charade indicator (letters next to each other).
'essential character' indicates the central letters (the character at the middle or essence of the word).
The middle of 'theme' is 'e'.
'a'+'stair'+'e'='ASTAIRE'
'to become' acts as a link.
(Other definitions for astaire that I've seen before include "Fred, immortal Hollywood dancer" , "Fred, Hollywood dancing icon" , "Fred -, twentieth century US dancer" , "US dancer (Top Hat)" , "A satire about Fred, great Hollywood dancer" .)