Only barren places are what's merited (4,7)
I believe the answer is:
just deserts
'what's merited' is the definition.
I can't judge whether this defines the answer.
'only barren places' is the wordplay.
'only' becomes 'just' (synonyms).
'barren places' becomes 'deserts' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'just'+'deserts'='JUST DESERTS'
'are' is the link.
Can you help me to learn more?
(Other definitions for just deserts that I've seen before include "Fate or results one deserves" , "Fair fate - rightful reward" , "What one has coming" .)