Sharp cry (4)
'sharp cry' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for yelp that I've seen before include "Short sharp cry (of such as a dog)" , "(Eg, dog's) short cry of alarm" , "Express discomfort" , "Sharp shrill cry or bark" , "Sharp shrill cry of dog (4)" .)
(Other definitions for keen that I've seen before include "Razor-edged" , "devoted" , "Eager - to lament the dead" , "Acute" , "Eager; dirge" .)