Country conserving head of prehistoric freshwater creature (8)
I believe the answer is:
terrapin
'freshwater creature' is the definition.
(animal that lives in water)
'country conserving head of prehistoric' is the wordplay.
'country' becomes 'terrain' (I've seen this before).
'conserving' indicates putting letters inside.
'head of' suggests taking the first letters.
The first letter of 'prehistoric' is 'p'.
'terrain' going around 'p' is 'TERRAPIN'.
(Other definitions for terrapin that I've seen before include "Partner I found for freshwater tortoise" , "A reptile; a printer (anag.)" , "Swimmer" , "Aquatic tortoise" , "edible, freshwater reptile" .)