Watches set of programmes about National Trust (8)
I believe the answer is:
sentries
'watches' is the definition.
('sentry' can be a synonym of 'watch')
'set of programmes about national trust' is the wordplay.
'set of programmes' becomes 'series' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' is an insertion indicator.
'national trust' becomes 'nt'.
'series' enclosing 'nt' is 'SENTRIES'.
Can you help me to learn more?
(Other definitions for sentries that I've seen before include "Lookouts, guards" , "Guards to control entry" , "Soldiers on guard duty" , "Sentinels, guards" , "It sneers at those on watch" .)