The complaint of a fathead, always? (5)
I believe the answer is:
fever
'the complaint' is the definition.
'fathead always?' is the wordplay.
'fathead' becomes 'f' (head letter of 'fat').
'always?' becomes 'ever' ('ever' can be a synonym of 'always').
'f'+'ever'='FEVER'
'of a' is the link.
(Other definitions for fever that I've seen before include "Reason for temperature" , "Masefield's desire for the sea" , "excited state" , "Intense nervous excitement or anticipation?" , "Excitement; ague" .)