Small group of married couples with time for midnight rendezvous (6)

'midnight rendezvous' is the definition.
(term for a satanic gathering)

'small group of married couples with time' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'group of married couples' becomes 'ABBA' (band featuring two married couples).
'with' says to put letters next to each other.
'time' becomes 't' (abbreviation).
's'+'abba'+'t'='SABBAT'

'for' is the link.