Beginning to crumble stone (5)
I believe the answer is:
onset
'beginning' is the definition.
(I know that onset is a type of start)
'crumble stone' is the wordplay.
'crumble' indicates an anagram.
'stone' is an anagram of 'ONSET'.
'to' acts as a link.
(Other definitions for onset that I've seen before include "Cast stone in attack" , "Beginning (of something often unpleasant)" , "Appearance of" , "Storming" , "Attack using stone" .)