Rocks King in mixed doubles (8)
I believe the answer is:
boulders
'rocks' is the definition.
(boulder is a kind of rock)
'king in mixed doubles' is the wordplay.
'king' becomes 'R' (abbreviation of Latin rex).
'in' is an insertion indicator.
'mixed' indicates an anagram.
'doubles' with letters rearranged gives 'bouldes'.
'r' placed into 'bouldes' is 'BOULDERS'.
(Other definitions for boulders that I've seen before include "Large smooth rocks" , "Huge 17s" , "Large pieces of rock worn smooth by weathering" , "Rocks larger than cobbles" , "Large masses of rock" .)