A gumboil no one treated leads to complaint (7)
I believe the answer is:
lumbago
'complaint' is the definition.
(I've seen this before)
'a gumboil no one treated' is the wordplay.
'no' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'treated' is an anagram indicator.
'a'+'gumboil'='agumboil'
'agumboil' with 'i' taken away is 'agumbol'.
'agumbol' with letters rearranged gives 'LUMBAGO'.
'leads to' acts as a link.
(Other definitions for lumbago that I've seen before include "Maul bog badly for a rheumatic affliction" , "Rheumatic backache" , "Lower back ache" , "Pain -- a bum log (anag)" , "Rheumatic pain in muscles of lower back" .)