Giant in unlikely story? (7)
I believe the answer is:
whopper
'story?' is the definition.
(both can mean a lie)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for whopper that I've seen before include "Mammoth story" , "large portion?" , "Giant thing" , "Very large specimen" , "Huge thing - huge porky?" .)