Restaurant in pub is trouble (6)
I believe the answer is:
bistro
'restaurant' is the definition.
(bistro is a kind of restaurant)
'in pub is trouble' is the wordplay.
'in' indicates the answer is hidden within the clue.
'BISTRO' is hidden in the letters of 'pub is trouble'.
(Other definitions for bistro that I've seen before include "A small informal restaurant from France" , "Eating place" , "Little restaurant" , "Small bar-cum-restaurant" , "Small inexpensive eatery" .)