Quantity of current illuminated Cornish resort, but not foremost expanse (9)
I believe the answer is:
amplitude
'expanse' is the definition.
(thesaurus)
'quantity of current illuminated cornish resort but not foremost' is the wordplay.
I cannot quite see how this works, but
'quantity' could be 'amp' (amp is a kind of quantity) and 'amp' is located in the answer.
'current' could be 'i' (symbol for electric current) and 'i' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for amplitude that I've seen before include "Range" , "Profusion" , "Abundance, largeness" , "stretch" , "Maximum extent of signal or vibration" .)