They have a definite stage in their lives (6)
I believe the answer is:
actors
'lives' is the definition.
The answer and definition can be both people as well as being plural nouns.
Perhaps they are linked in a way I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for actors that I've seen before include "Players on stage" , "Thespians" , "Theatrical performers" , "Luvvies" , "They 23 down" .)