The final despair is attributed to them (5)
I believe the answer is:
their
'them' is the definition.
I know nothing about this answer so I can't tell whether this works.
'the final despair is attributed to' is the wordplay.
'final despair' becomes 'ir' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'is attributed to' says to put letters next to each other.
'the'+'ir'='THEIR'
Can you help me to learn more?
(Other definitions for their that I've seen before include "Possessive pronoun" , "Belonging to those people" , "Those people's" , "Belonging to him (perhaps wrongly)" , "belonging to an individual" .)