It's unfortunate only the last of fliers is accepted, in truth (9)
I believe the answer is:
adversity
'it's' is the definition.
I can't judge whether this definition defines the answer.
'unfortunate only the last of fliers is accepted in truth' is the wordplay.
'unfortunate only' becomes 'ad' (I am not sure about this - if you are sure you should believe this answer much more).
'the last of' indicates one should take the final letters.
'is accepted in' means one lot of letters goes inside another (I've seen this in other clues).
'truth' becomes 'verity' (verity is a kind of truth).
The final letter of 'fliers' is 's'.
's' placed within 'verity' is 'versity'.
'ad'+'versity'='ADVERSITY'
Can you help me to learn more?
(Other definitions for adversity that I've seen before include "Circumstances that cause trouble or sorrow" , "Hard times or misfortune" , "Misfortune, hardship" , "State of misfortune or affliction" .)