Decorate or give title to National Trust (8)
I believe the answer is:
ornament
'decorate' is the definition.
('ornament' can be a synonym of 'decorate')
'or give title to national trust' is the wordplay.
'give' means one lot of letters go next to another.
'title' becomes 'name' (I've seen this before).
'to national trust' becomes 'nt'.
'or'+'name'+'nt'='ORNAMENT'
(Other definitions for ornament that I've seen before include "Decorate" , "Something decorative" , "Embellish" , "deck" , "Decorative item" .)