Sole relative climbing mountain rocks (6)
I believe the answer is:
plaice
'sole relative' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'climbing mountain rocks' is the wordplay.
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'mountain' becomes 'alp' (the Alps are a mountain range).
'rocks' becomes 'ice' (both can informally mean diamonds).
'alp' in reverse letter order is 'pla'.
'pla'+'ice'='PLAICE'
Can you help me to learn more?
(Other definitions for plaice that I've seen before include "Location of the fish" , "Flatfish, valued as food" , "The fish sounds as if it hits the spot" , "Large flat fish" , "A fish; sounds like put" .)