Whole country is full of conservationists (6)
I believe the answer is:
entire
'whole' is the definition.
('entire' can be similar in meaning to 'whole')
'country is full of conservationists' is the wordplay.
'country' becomes 'eire' (Eire is an example).
'is full of' indicates putting letters inside.
'conservationists' becomes 'nt' (abbreviation for National Trust).
'eire' going around 'nt' is 'ENTIRE'.
(Other definitions for entire that I've seen before include "Complete, intact" , "Whole and complete" , "Whole; unbroken" , "Full, intact" , "Unmitigated" .)