Deplore weak conservationists (6)
I believe the answer is:
lament
'deplore' is the definition.
(I've seen this before)
'weak conservationists' is the wordplay.
'weak' becomes 'lame' ('lame' can be similar in meaning to 'weak').
'conservationists' becomes 'nt' (abbreviation for National Trust).
'lame'+'nt'='LAMENT'
(Other definitions for lament that I've seen before include "Show of sorrow" , "Wail" , "Mourning song or poem" , "Express sorrow" , "Elegy" .)