Fever may produce first-class alarm (7)
I believe the answer is:
malaria
'fever' is the definition.
(I've seen this before)
'first-class alarm' is the wordplay.
I cannot really see how this works, but
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is located in the answer.
an anagram of 'alarm' is 'malar' which is within the answer.
A single letter 'a' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'may produce' acts as a link.
Can you help me to learn more?
(Other definitions for malaria that I've seen before include "Mosquito-transmitted disease" , "Fever-inducing disease" , "Disease causing fever transmitted by mosquitos" , "Disease spread by mosquito bites" , "Infectious fever" .)