Seldom coming from bank, crossing a river (6)
I believe the answer is:
rarely
'seldom' is the definition.
(I know that seldom can be written as rarely)
'bank crossing a river' is the wordplay.
'bank' becomes 'rely' (to bank on is to rely on).
'crossing' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'a'+'r'='ar'
'rely' enclosing 'ar' is 'RARELY'.
'coming from' is the link.
(Other definitions for rarely that I've seen before include "very well" , "Not very often, seldom in fact" , "once in a blue moon" , "Seldom, not frequently" , "when hundred-to-one horse wins?" .)