As some rock relatively still, I'm breaking it (11)
I believe the answer is:
sedimentary
'as some rock' is the definition.
(like sedimentary rock)
'relatively still i'm breaking it' is the wordplay.
'relatively still' becomes 'sedentary' (sedentary means generally sitting down).
'breaking it' indicates putting letters inside (inserted letters break in).
'sedentary' going around 'im' is 'SEDIMENTARY'.
(Other definitions for sedimentary that I've seen before include "Like some rocks" , "grounds" , "having been deposited here" , "Describing a type of rock" , "(Of rock) laid down by deposition" .)