Young woman crossing river with festoon of flowers (7)
I believe the answer is:
garland
'festoon of flowers' is the definition.
(I've seen this before)
'young woman crossing river with' is the wordplay.
'young woman' becomes 'gal' ('gal' can be a synonym of 'young woman').
'crossing' indicates putting letters inside.
'river' becomes 'r'.
'with' becomes 'and' (I've seen this before**).
'gal' going around 'r' is 'garl'.
'garl'+'and'='GARLAND'
(Other definitions for garland that I've seen before include "Festoon of flowers" , "Deck" , "Judy - - , legendary singer" , "Floral wreath" , "Judy . . . . . . . who played Dorothy in The Wizard of Oz" .)