River creature with weapon creates unnecessary panic (5,5)
I believe the answer is:
false alarm
'unnecessary panic' is the definition.
(a false alarm is an unnecessary panic)
'river creature with weapon' is the wordplay.
'river' becomes 'fal' (I've seen this before).
'creature' becomes 'seal' (type of animal).
'with' means one lot of letters go next to another.
'weapon' becomes 'arm' (synonyms).
'fal'+'seal'+'arm'='FALSE ALARM'
'creates' is the link.
(Other definitions for false alarm that I've seen before include "Panic over" , "Warning proving unnecessary" , "unfounded scare?" .)