Son during lunch perhaps expresses hollow complaint (7)
I believe the answer is:
measles
'complaint' is the definition.
(measles is a kind of health problem)
'son during lunch perhaps expresses hollow' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'during' means one lot of letters goes inside another.
'lunch perhaps' becomes 'meal' (lunch is a kind of meal).
'hollow' says to hollow out the word (remove centre letters) (the word is hollowed out).
'expresses' with its middle taken out is 'es'.
's' inserted inside 'meal' is 'measl'.
'measl'+'es'='MEASLES'
(Other definitions for measles that I've seen before include "Rash-causing disease" , "Childhood malady" , "Children's disease with red spots" , "Infection marked by red spots" , "Disease suffered in childhood" .)