Go in with Sue smashing type of rock (7)
I believe the answer is:
igneous
'type of rock' is the definition.
(I've seen this before)
'go in with sue smashing' is the wordplay.
'with' means one lot of letters go next to another.
'smashing' indicates anagramming the letters.
'go'+'in'+'sue'='goinsue'
'goinsue' with letters rearranged gives 'IGNEOUS'.
(Other definitions for igneous that I've seen before include "(Of rock) formed from lava or magma" , "Of fire, fiery" , "Maybe fiery" , "Formed of solidified magma" , "(Rock) of volcanic origin" .)