Sauce eaten with fish (7)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
tartare
(Other definitions for tartare that I've seen before include "Type of mayonnaise sauce" , "(Mayonnaise) sauce; raw (steak)" , "Served raw, steak say" , "Mayonnaise-based sauce with capers, eaten with fish" , "Mayonnaise dressing served with fish" .)