Complaint I will bring to a head (7)
I believe the answer is:
illness
'complaint' is the definition.
(thesaurus)
'i will bring to a head' is the wordplay.
'i will' becomes 'ill' (I've seen this before).
'bring to a' says to put 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" .)