Stand in part of theatre, including nationally acclaimed trio, initially (8)
I believe the answer is:
stagnate
'stand' is the definition.
(stand can mean to stagnate or become still)
'part of theatre including nationally acclaimed trio initially' is the wordplay.
'part of theatre' becomes 'stage' ('stage' is part of 'theatre').
'including' means one lot of letters goes inside another.
'initially' says to take the initial letters.
The first letters of 'nationally acclaimed trio' is 'nat'.
'stage' placed around 'nat' is 'STAGNATE'.
'in' acts as a link.
(Other definitions for stagnate that I've seen before include "remain motionless" , "Fester" , "resemble pond water?" , "Tan gates could vegetate" , "turn foul" .)