Firm conservationists inspiring casual worker’s disdain (8)
I believe the answer is:
contempt
'disdain' is the definition.
('contempt' can be a synonym of 'disdain')
'firm conservationists inspiring casual worker's' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'conservationists' becomes 'nt' (abbreviation for National Trust).
'inspiring' indicates putting letters inside.
'casual worker' becomes 'temp' (I've seen this before).
'nt' going around 'temp' is 'ntempt'.
'co'+'ntempt'='CONTEMPT'
(Other definitions for contempt that I've seen before include "Feeling of disdain, sometimes of court" , "Offence of disrespect in court" , "Disdain or disrespect, perhaps or court" , "Disdain, say, perhaps of court" , "Wilful disrespect of authority" .)