One in desperate need of help by Friday? (8)
I believe the answer is:
castaway
'friday?' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'one in desperate need of help by' is the wordplay.
'one in desperate need of help' becomes 'cast' (I can't explain this - if you can you should give a lot more credence to this answer).
'by' becomes 'away' (synonyms).
'cast'+'away'='CASTAWAY'
Can you help me to learn more?
(Other definitions for castaway that I've seen before include "one flung out" , "Type of lace" , "Shipwrecked character" , "Eg, Robinson Crusoe" , "Shipwrecked or abandoned person like Robinson Crusoe" .)