Notice present provided by National Trust supporter (8)
I believe the answer is:
adherent
'supporter' is the definition.
(I've seen this in another clue)
'notice present provided by national trust' is the wordplay.
'notice' becomes 'ad' (both can mean an advertisement).
'present' becomes 'here' (both can mean in attendance).
'provided by' says to put letters next to each other.
'national trust' becomes 'nt'.
'ad'+'here'+'nt'='ADHERENT'
(Other definitions for adherent that I've seen before include "Dedicated supporter" , "One supporting" , "Freak" , "Supporter, follower" , "Person supporting" .)