Allowed actor initially to enter stage left (5)
I believe the answer is:
legal
'allowed' is the definition.
(I know that allowed can be written as legal)
'actor initially to enter stage left' is the wordplay.
'initially' indicates taking the first letters.
'to enter' means one lot of letters goes inside another.
'stage' becomes 'leg' (a leg is a stage of a competition).
'left' becomes 'l' (common abbreviation).
The first letter of 'actor' is 'a'.
'leg'+'l'='legl'
'a' put inside 'legl' is 'LEGAL'.
(Other definitions for legal that I've seen before include "Legitimate" , "Like some tender" , "Rightful" , "Allowed by law" , "Above board" .)