Usher in National Theatre following a dry finale with a bit of applause (9)
I believe the answer is:
attendant
'usher in national' is the definition.
'attendant' can be an answer for 'usher' (thesaurus). I'm not certain of the 'in national' bit.
'theatre following a dry finale with a bit of applause' is the wordplay.
'theatre' becomes 'nt' (National Theatre).
'following' is a charade indicator (letters next to each other).
'a dry finale with' becomes 'attend' (I can't explain this - if you can you should believe this answer much more).
'a bit of' suggests taking the first letters.
The first letter of 'applause' is 'a'.
'attend'+'a'='attenda'
'nt' after 'attenda' is 'ATTENDANT'.
(Other definitions for attendant that I've seen before include "Related" , "One present to guide, help" , "S/he sees to the needs of others" , "waiter" , "Person present as companion or servant" .)