Lead actor must take one step (5)
I believe the answer is:
stair
'step' is the definition.
(a single step in a staircase)
'lead actor must take one' is the wordplay.
'lead actor' becomes 'star' (eg the star of a show).
'must take' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'star' placed around 'i' is 'STAIR'.
(Other definitions for stair that I've seen before include "this could get one down" , "One of a 15" , "single step in a series" , "One of a flight of steps" , "section of flight" .)