Trumpet in concert stuffed with filling of long cake? (8)
I believe the answer is:
proclaim
'trumpet' is the definition.
(thesaurus)
'concert stuffed with filling of long cake?' is the wordplay.
'concert' becomes 'prom' (abbreviation for promenade concert).
'stuffed with' is an insertion indicator (some letters get stuffed inside).
'filling of' indicates the central letters (the letters which fill the inside of the word).
'long cake?' becomes 'eclair' (type of cake).
The centre of 'eclair' is 'clai'.
'prom' enclosing 'clai' is 'PROCLAIM'.
'in' acts as a link.
(Other definitions for proclaim that I've seen before include "advertise" , "shout from the housetops" , "Declare formally" , "Reveal" , "Speak out" .)