Complaint I will bring to a head (7)
I believe the answer is:
illness
'complaint' is the definition.
(illness is a kind of health problem)
'i will bring to a head' is the wordplay.
'i will' becomes 'I'll' (abbreviation).
'bring to a' is a charade indicator (letters next to each other) (bring some letters to others).
'head' becomes 'ness' (ness is a headland).
'ill'+'ness'='ILLNESS'
(Other definitions for illness that I've seen before include "Malady, sickness" , "Sick condition" , "Ailment, disease" , "Period of sickness" , "Upset" .)