Was his aversion to fat doubly fishy? (4,5)
I believe the answer is:
jack sprat
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for jack sprat that I've seen before include "One leaving fat" , "he stuck to a fat-free diet" , "He ate no fat" , "He ate only lean" , "Nursery-rhyme character" .)