Review site (4)
I believe the answer is:
yelp
'review site' is the definition.
(I know that Yelp is a review site)
This is the entire clue.
(Other definitions for yelp that I've seen before include "Sharp shrill cry of dog (4)" , "Painful sound" , "Short sharp cry (of such as a dog)" , "(Eg, dog's) short cry of alarm" , "Bark like a puppy" .)