Where fertility is exceptional (5)
I believe the answer is:
oasis
'where fertility' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'is exceptional' is the wordplay.
I cannot really see how this works, but
'is' is found in the answer.
The remaining letters 'oas' 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).
Can you help me to learn more?
(Other definitions for oasis that I've seen before include "Safe haven" , "Fertile area in desert" , "Feature of desert" , "Noel & Liam's band" , "Fertile tract in a desert" .)