Hissing excuses upset conservation group (8)
I believe the answer is:
sibilant
'hissing' is the definition.
(I've seen this in another clue)
'excuses upset conservation group' is the wordplay.
'excuses' becomes 'alibis' (alibiing is a kind of excusing).
'upset' says the letters should be written in reverse.
'conservation group' becomes 'NT' (abbreviation for National Trust).
'alibis' written backwards gives 'sibila'.
'sibila'+'nt'='SIBILANT'
(Other definitions for sibilant that I've seen before include "Having a hissing sound as in speech" , "Consonant with a hissing sound" .)