Forget somehow to describe extremely rare amphibian (4,4)
I believe the answer is:
tree frog
'amphibian' is the definition.
(I know that tree frog is a type of anuran)
'forget somehow to describe extremely rare' is the wordplay.
'somehow' indicates anagramming the letters.
'to describe' is an insertion indicator.
'extremely' suggests removing the centre (only the extremes of the word are used).
'rare' with its centre removed is 're'.
'forget' anagrammed gives 'tefrog'.
'tefrog' placed around 're' is 'TREE-FROG'.
(Other definitions for tree frog that I've seen before include "High jumper" , "Amphibian found in the branches?" , "Arboreal amphibian" .)