Tree from which swan crosses a river (5)
I believe the answer is:
carob
'tree' is the definition.
(carob is a kind of tree)
'swan crosses a river' is the wordplay.
'swan' becomes 'cob' (cob is a kind of swan).
'crosses' indicates putting letters inside.
'river' becomes 'r'.
'a'+'r'='ar'
'cob' enclosing 'ar' is 'CAROB'.
'from which' is the link.
(Other definitions for carob that I've seen before include "that one could use for chocolate" , "(Seeds yielding) powder used as a chocolate substitute" , "Chocolate substitute pod" , "St John's bread" , "Saint John's bread" .)