Lies about National Trust endowments (7)
I believe the answer is:
talents
'endowments' is the definition.
('talent' can be a synonym of 'endowment')
'lies about national trust' is the wordplay.
'lies' becomes 'tales' (tale is a kind of lie).
'about' is an insertion indicator.
'national trust' becomes 'nt'.
'tales' placed around 'nt' is 'TALENTS'.
(Other definitions for talents that I've seen before include "Special aptitudes" , "Gifts, abilities" , "Natural aptitudes" , "Last ten (anag.)" , "endowments" .)