Amazing party after try a French fellow secured (7-2)
I believe the answer is:
unheard-of
'amazing' is the definition.
(something unheard of is amazing or rare)
'party after try a french fellow secured' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'after' means one lot of letters go next to another.
'try' becomes 'hear' (as in to try someone in court or hear their case).
'a french fellow' becomes 'unf' (I can't justify this - if you can you should believe this answer much more).
'secured' indicates putting letters inside (inserted letters are secured inside).
'do' after 'hear' is 'heardo'.
'heardo' placed within 'unf' is 'UNHEARD-OF'.
(Other definitions for unheard-of that I've seen before include "Inconceivable" , "Previously unknown" , "Fear hound being not previously known" , "Without precedent" , "Unknown - novel" .)