Handel wrote one speech mostly to entertain troops (8)
I believe the answer is:
oratorio
'handel wrote one' is the definition.
(George Frideric Handel composed several oratorios)
'speech mostly to entertain troops' is the wordplay.
'speech' becomes 'oration' (oration is a kind of speech).
'mostly' means to remove the last letter.
'to entertain' means one lot of letters goes inside another.
'troops' becomes 'or' (military abbreviation for other ranks).
'oration' with its final letter taken away is 'oratio'.
'oratio' enclosing 'or' is 'ORATORIO'.
(Other definitions for oratorio that I've seen before include "Musical composition for voices and orchestra based on a religious text" , "Sacred, dramatic music" , "Musical work for orchestra and voices" , "Music for voices and orchestra with biblical theme" , "Large choral work" .)