Gatsby was one in flipping pickle over RTA primarily (5)
I believe the answer is:
major
'gatsby was one' is the definition.
I can't judge whether this defines the answer.
'flipping pickle over rta primarily' is the wordplay.
'flipping' says the letters should be written backwards.
'pickle' becomes 'jam' (both can mean a problem or difficulty).
'primarily' suggests taking the first letters.
The initial letters of 'over rta' is 'or'.
'jam' reversed gives 'maj'.
'maj'+'or'='MAJOR'
'in' is the link.
Can you help me to learn more?
(Other definitions for major that I've seen before include "Army rank" , "course for American students" , "Important; old Animal Farm boar" , "Chief - officer" , "Bigger" .)