Informal French restaurant (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
bistro
'informal french restaurant' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bistro that I've seen before include "Little restaurant" , "Small informal french-style resturaunt" , "family concern perhaps" , "Cheap restaurant" , "'Small, inexpensive restaurant (6)'" .)