'I seek to impress,' you said, cracking Toughie (6)
I believe the answer is:
poseur
'i' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'seek to impress you said cracking toughie' is the wordplay.
I cannot quite see how this works, but
'you' could be 'u' (commonly used in online communication) and 'u' is located in the answer.
'toughie' could be 'poser' (synonyms) and 'poser' is found within the leftover letters.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for poseur that I've seen before include "Pseud, posturer" , "Uprose (anag.)" , "One who strikes attitudes, a phoney" , "Person who strikes attitudes, a show-off" , "Pretentious type" .)