Tiny tree developing — it never stops (8)
I believe the answer is:
eternity
'it never stops' is the definition.
(eternity goes on forever)
'tiny tree developing' is the wordplay.
'developing' indicates an anagram (letters developed into a new form).
'tiny'+'tree'='tinytree'
'tinytree' anagrammed gives 'ETERNITY'.
(Other definitions for eternity that I've seen before include "Hereafter" , "Unending time" , "Jean-Paul --, French author" , "Endless time" , "destination of film?" .)