Firm conservationists accepting casual worker's ridicule (8)
I believe the answer is:
contempt
'ridicule' is the definition.
(I've seen this before)
'firm conservationists accepting casual worker's' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'conservationists' becomes 'NT' (abbreviation for National Trust).
'accepting' means one lot of letters goes inside another (inserted letters are accepted).
'casual worker' becomes 'temp' (short-term or temporary worker).
'nt' going around 'temp' is 'ntempt'.
'co'+'ntempt'='CONTEMPT'
(Other definitions for contempt that I've seen before include "ridicule" , "Open disrespect, to a court, say" , "In which you may be held" , "Complete lack of respect, as sometimes of court" , "Feeling of disdain, sometimes of court" .)