Fellow left union in complaint (3)
I believe the answer is:
flu
'complaint' is the definition.
(flu is a kind of disease)
'fellow left union' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'left' becomes 'l' (common abbreviation).
'union' becomes 'u' (common abbreviation as in 'Trades Union Congress').
'f'+'l'+'u'='FLU'
'in' acts as a link.
(Other definitions for flu that I've seen before include "Minor ailment" , "Viral infection (colloq.)" , "Swine --; gastric --" , "Winter ailment" , "Winter disease, awfully avian" .)