Pit in Devonshire drained by conservationists (4)
I believe the answer is:
dent
'pit' is the definition.
(thesaurus)
'devonshire drained by conservationists' is the wordplay.
'drained' means to remove the middle letters.
'by' says to put letters next to each other.
'conservationists' becomes 'nt' (abbreviation for National Trust).
'devonshire' with its middle removed is 'de'.
'de'+'nt'='DENT'
'in' is the link.
(Other definitions for dent that I've seen before include "Hollow, depression" , "Surface hollow" , "Tend to small indentation" , "tooth (13) [FRENCH]" , "Notch" .)