Pit in Downe drained by National Trust (4)
I believe the answer is:
dent
'pit' is the definition.
(thesaurus)
'downe drained by national trust' is the wordplay.
'drained' says to hollow out the word (remove centre letters).
'by' is a charade indicator (letters next to each other).
'national trust' becomes 'nt'.
'downe' with its centre taken out is 'de'.
'de'+'nt'='DENT'
'in' acts as a link.
(Other definitions for dent that I've seen before include "Hollow, depression" , "Injury to pride" , "tooth (13) [FRENCH]" , "Slight damage" , "Hollow caused by a blow" .)