Abrupt turn of phrase used by jerk is fatuous (7)
I believe the answer is:
idiotic
'fatuous' is the definition.
(I've seen this before)
'abrupt turn of phrase used by jerk is' is the wordplay.
I cannot quite see how this works, but
'turn' could be 't' (abbreviation in Oxford Dictionary of Abbreviations) and 't' is located in the answer.
'of' could be 'o' and 'o' is located in the answer.
'is' could be 'ii' (more than one I) and 'ii' is found within the leftover letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for idiotic that I've seen before include "half-baked" , "Silly" , "without intelligence" , "without any point" , "Fatuous" .)