Treading the boards (7)
I believe the answer is:
'treading the boards' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for onstage that I've seen before include "as a player may be?" , "performing in the theatre?" , "Visible to (theatre) audience" , "in full view of the audience" , "Visible to theatregoers" .)