Old woman’s ultimately serious condition, a rocky situation (6)
I believe the answer is:
massif
'a rocky situation' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'old woman's ultimately serious condition' is the wordplay.
'old woman's' becomes 'ma's' (belongs to ma).
'ultimately' suggests the final letters.
'condition' becomes 'if' (as in 'a big if').
The last letter of 'serious' is 's'.
'mas'+'s'+'if'='MASSIF'
Can you help me to learn more?
(Other definitions for massif that I've seen before include "French mountain range" , "Extensive raised area" , "Geological formation of an IMF ass" , "Geographical formation" , "highlands" .)