Big fib (7)
I believe the answer is:
whopper
'big fib' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for whopper that I've seen before include "Humongous untruth" , "In slang, it's very big - like a lie or fish" , "Giant pork pie?" , "Huge one; lie" , "A big one, lie or fish" .)