A National Trust insect (3)
I believe the answer is:
ant
'insect' is the definition.
(ant is a kind of insect)
'a national trust' is the wordplay.
'national trust' becomes 'nt'.
'a'+'nt'='ANT'
(Other definitions for ant that I've seen before include "Hill-dweller" , "A soldier, for one" , "Small social insect" , "one living in colonies" , "colonist" .)