'Amusing' conservationists pinching lady's bum (9)
I believe the answer is:
fundament
'bum' is the definition.
('fundament' can be a synonym of 'bum')
'amusing conservationists pinching lady's' is the wordplay.
'amusing' becomes 'fun' ('fun' can be a synonym of 'amusing').
'conservationists' becomes 'nt' (abbreviation for National Trust).
'pinching' indicates putting letters inside (taking inside).
'lady' becomes 'dame' ('dame' can be a synonym of 'lady'**).
'fun'+'nt'='funnt'
'funnt' going around 'dame' is 'FUNDAMENT'.
(Other definitions for fundament that I've seen before include "Underlying basis - foundation" , "seat" , "bottom" .)