Once trendy, you French admitted being agitated (3,2)
I believe the answer is:
het up
'being agitated' is the definition.
(I've seen this before)
'once trendy you french admitted' is the wordplay.
'once trendy' becomes 'hep' (I've seen this in other clues. I am not sure about the 'once' bit.**).
'you french' becomes 'tu' (I've seen this in other clues).
'admitted' indicates putting letters inside.
'hep' placed around 'tu' is 'HET UP'.
Can you help me to learn more?
(Other definitions for het up that I've seen before include "in a rage?" , "On edge - agitated" , "Indignant" , "Upset, agitated" , "Overwrought" .)