The runs -- sickness to do with raised temperature (7)
I believe the answer is:
thermal
'do with raised temperature' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'the runs sickness' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'sickness' becomes 'mal' (I've seen this before).
'the'+'r'+'mal'='THERMAL'
'to' is the link.
Can you help me to learn more?
(Other definitions for thermal that I've seen before include "Warm or hot, say air or spring" , "A lot of hot air" , "Rising current of warm air" , "Concerned with high temperature" , "Up-current of air" .)