National Trust engages a certain ensemble (5)
I believe the answer is:
nonet
'ensemble' is the definition.
(I know that nonet is a type of ensemble)
'national trust engages a certain' is the wordplay.
'national trust' becomes 'nt'.
'engages' is an insertion indicator.
'a certain' becomes 'one' (ie one particular example of something).
'nt' enclosing 'one' is 'NONET'.
(Other definitions for nonet that I've seen before include "Multi-figure composition" , "Instrumental group of nine players" , "Tonne (anag.); music for a group" , "Composition for nine performers" , "Piece for nine musicians" .)