Lassie bit cracked root (9)
I believe the answer is:
stabilise
'root' is the definition.
(I know this)
'lassie bit cracked' is the wordplay.
'cracked' is an anagram indicator.
'lassie'+'bit'='lassiebit'
'lassiebit' anagrammed gives 'STABILISE'.
(Other definitions for stabilise that I've seen before include "Make of keep steady or balanced" , "regulate equilibrium" , "Stop (something) rocking" , "Become less changeable" , "Fix; regulate" .)