Lady Fortune, according to Spooner, is a weakling (4,4)
I believe the answer is:
lame duck
'a weakling' is the definition.
(weak or ineffectual person)
'lady fortune according to spooner' is the wordplay.
I cannot quite understand how this works, but
'lady' could be 'ame' and 'ame' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for lame duck that I've seen before include "Feeble person" , "Someone no longer able to perform effectively" , "Person in need of help, like footsore mallard" , "Someone depending heavily on the help of others" , "Inefficient or helpless person" .)