Complaint as Frenchman relaxes around lake (7)
I believe the answer is:
measles
'complaint' is the definition.
(measles is a kind of disease)
'frenchman relaxes around lake' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'relaxes' becomes 'eases' (ease can mean to relax or reduce).
'around' is an insertion indicator.
'lake' becomes 'L' (geographical abbreviation).
'eases' enclosing 'l' is 'easles'.
'm'+'easles'='MEASLES'
'as' is the link.
(Other definitions for measles that I've seen before include "Disease suffered in childhood" , "Infectious disease covering body with red spots" , "Spotty viral infection" , "Viral disease once common in children" , "Rash-causing disease" .)