Express sorrow, not convincing conservation group (6)
I believe the answer is:
lament
'express sorrow' is the definition.
(I've seen this before)
'convincing conservation group' is the wordplay.
'convincing' becomes 'lame' (I can't explain this - if you can you should believe this answer much more).
'conservation group' becomes 'NT' (abbreviation for National Trust).
'lame'+'nt'='LAMENT'
'not' is the link.
(Other definitions for lament that I've seen before include "Elegy" , "Expression of sorrow" , "Bemoan, bewail" , "Cry of sorrow" , "Mourn in mental way (6)" .)