A rock to ramble round? (6)
I believe the answer is:
marble
'a rock' is the definition.
(I know that marble is a type of metamorphic rock)
'ramble round?' is the wordplay.
'round?' indicates an anagram (I've seen 'round' mean this).
'ramble' with letters rearranged gives 'MARBLE'.
'to' is the link.
(Other definitions for marble that I've seen before include "Fine stone" , "Alley possibly" , "Glass ball toy" , "Hard crystalline limestone, can be polished" , "Hard crystalline metamorphic rock" .)