Rock setter’s stuffing in case 1 (9)
I believe the answer is:
limestone
'rock' is the definition.
(limestone is a kind of rock)
'setter's stuffing in case 1' is the wordplay.
'setter's' becomes 'im' (I am - referring to the crossword setter).
'stuffing' is an insertion indicator.
'in case' becomes 'lest' (as in 'lest we forget').
'i' becomes 'one' (synonyms).
'im' put inside 'lest' is 'limest'.
'limest'+'one'='LIMESTONE'
(Other definitions for limestone that I've seen before include "Clint" , "Mineral" , "Rock of calcium carbonate" , "Tie lemons up in sedimentary rock" , "Caves in here perhaps" .)