Thankful it's about piece of fiction by retiring old leader (8)
I believe the answer is:
relieved
'thankful' is the definition.
(I've seen this before)
'about piece of fiction by retiring old leader' is the wordplay.
'about' means one lot of letters goes inside another.
'piece of fiction' becomes 'lie' (I've seen this before).
'by retiring old' becomes 've' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'leader' becomes 'red' (red is a kind of leader).
'lie'+'ve'='lieve'
'lieve' going inside 'red' is 'RELIEVED'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for relieved that I've seen before include "Replaced on duty" , "Raised the siege of, or generally eased pressure" , "Freed from anxiety or duty" , "Released from duty or freed from worry" , "Freed by a substitute? - that relaxed the tension" .)