Crossing river, crash merits my angry greeting (5,9)
I believe the answer is:
merry christmas
'greeting' is the definition.
(I've seen this in another clue)
'crossing river crash merits my angry' is the wordplay.
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation for river).
'angry' indicates an anagram.
'crash'+'merits'+'my'='crashmeritsmy'
'crashmeritsmy' is an anagram of 'merychristmas'.
'r' placed into 'merychristmas' is 'MERRY CHRISTMAS'.
(Another definition for merry christmas that I've seen is " festive greeting".)