Pressure to withdraw conservationists? It's happened before (9)
I believe the answer is:
precedent
'it's happened before' is the definition.
(something which has happened before)
'pressure to withdraw conservationists?' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'to withdraw' becomes 'recede' (synonyms).
'conservationists?' becomes 'NT' (abbreviation for National Trust).
'p'+'recede'+'nt'='PRECEDENT'
(Other definitions for precedent that I've seen before include "Example to follow" , "Going before" , "An earlier example serving as a guide" , "exemplary court case" , "Earlier parallel event" .)