A long time entertaining National Trust representatives (6)
I believe the answer is:
agents
'representatives' is the definition.
(agent is a kind of representative)
'a long time entertaining national trust' is the wordplay.
'a long time' becomes 'ages' (I've seen this before).
'entertaining' indicates putting letters inside.
'national trust' becomes 'nt'.
'ages' placed around 'nt' is 'AGENTS'.
(Other definitions for agents that I've seen before include "Representatives" , "They do business on behalf of others" , "travelling salesmen" , "Sales reps" , "Spies" .)