Marriages showing problems afoot? Not the first (6)
I believe the answer is:
unions
'marriages' is the definition.
('union' can be a synonym of 'marriage')
'problems afoot? not the first' is the wordplay.
'problems afoot?' becomes 'bunions' (a bunion is a bump on the foot).
'not the first' means to remove the first letter.
'bunions' with its first letter removed is 'UNIONS'.
'showing' acts as a link.
(Other definitions for unions that I've seen before include "Workers' associations" , "Marriages or workers' organisations" , "Couplings and labour organisations" , "Undoes a knot" , "Employee's organisations" .)