Cry upon seeing a mouse (3)
I believe the answer is:
eek
'cry upon seeing a mouse' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eek that I've seen before include "Cry of alarm" , "Alarmed expression" , "Expression of surprise" , "Yikes! Oo-er!" , "I'm frightened!" .)