Old piece of crust that's extremely dry (14)
I believe the answer is:
supercontinent
'old piece of crust' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'extremely dry' is the wordplay.
I cannot quite see how this works, but
'extremely' could be 'super' (synonyms) and 'super' is found in the answer.
The remaining letters 'continent' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'that's' acts as a link.
Can you help me to learn more?
(Another definition for supercontinent that I've seen is " Large early landmass".)