Small creatures not before seen climbing stone (5)
I believe the answer is:
newts
'small creatures' is the definition.
'newts' can be an answer for 'creatures' (newt is a kind of creature). I am not sure about the 'small' bit.
'not before seen climbing stone' is the wordplay.
'not before' becomes 'new' (I am not sure about this - if you are sure you should believe this answer much more).
'seen climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'stone' becomes 'st' (abbreviation).
'st' written backwards gives 'ts'.
'new'+'ts'='NEWTS'
Can you help me to learn more?
(Other definitions for newts that I've seen before include "Kind of water lizards or efts" , "creatures" , "Small amphibians, efts" .)