Old lady breaking layered rock, I see, causing splits (10)
I believe the answer is:
schismatic
'causing splits' is the definition.
(causing schisms)
'old lady breaking layered rock i see' is the wordplay.
'old lady' becomes 'ma' (both can informally mean mother).
'breaking' is an insertion indicator.
'layered rock' becomes 'schist' (I've seen this before).
'see' becomes 'C' (the word for the letter, according to Chambers).
'ma' going inside 'schist' is 'schismat'.
'schismat'+'i'+'c'='SCHISMATIC'
(Other definitions for schismatic that I've seen before include "Causing division in the RC Church" , "divisive character" , "Favouring separation from a church" , "fractious?" .)