Go over one detailed description penned by National Trust (7)
I believe the answer is:
inspect
'go over' is the definition.
('inspect' can be a synonym of 'go over')
'one detailed description penned by national trust' is the wordplay.
'one' becomes 'i' (Roman numeral).
'detailed description' becomes 'spec' (I've seen this before).
'penned by' indicates putting letters inside.
'national trust' becomes 'nt'.
'spec' placed within 'nt' is 'nspect'.
'i'+'nspect'='INSPECT'
(Other definitions for inspect that I've seen before include "Assess; examine" , "Scrutinise" , "Look at closely" , "Go over" , "Scrutinize, audit" .)