Rock etc a source of chalk (10)
I believe the answer is:
cretaceous
'of chalk' is the definition.
(I've seen this before)
'rock etc a source' is the wordplay.
'rock' is an anagram indicator (the solver must 'rock' the letters around).
'etc'+'a'+'source'='etcasource'
'etcasource' with letters rearranged gives 'CRETACEOUS'.
(Other definitions for cretaceous that I've seen before include "Chalky" , "of chalk" .)