App with restaurant reviews (4)
I believe the answer is:
yelp
'app with restaurant reviews' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for yelp that I've seen before include "Short sharp cry or bark" , "Little dog's bark" , "Sharp shrill cry of dog (4)" , "Tiny bark" , "Bark like a puppy" .)