Nth sortie amazingly and the most difficult? (9)
I believe the answer is:
thorniest
'the most difficult?' is the definition.
(thorny can mean difficult or complex)
'nth sortie amazingly' is the wordplay.
'amazingly' indicates anagramming the letters.
'nth'+'sortie'='nthsortie'
'nthsortie' with letters rearranged gives 'THORNIEST'.
'and' acts as a link.
(Other definitions for thorniest that I've seen before include "Most troublesome or prickly" , "most problematic" .)