The whole of humanity’s enthralled by overblown plot (9)
I believe the answer is:
allotment
'overblown plot' is the definition.
'allotment' can be an answer for 'plot' (I've seen this before). I am not certain of the 'overblown' bit.
'the whole of humanity's enthralled by' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'whole' becomes 'all' (I have seen 'The whole ' mean 'all' so perhaps 'whole' could also mean 'all').
'of' becomes 'o''.
'humanity' becomes 'ment' (I can't explain this - if you can you should believe this answer much more).
'enthralled by' indicates putting letters inside ('enthral' can mean 'capture').
'all'+'o'+'ment'='alloment'
't' placed within 'alloment' is 'ALLOTMENT'.
Can you help me to learn more?
(Other definitions for allotment that I've seen before include "Share - small piece of land rented for cultivation" , "Portion, distribution" , "Rented piece of ground" , "Plot of land rented for growing vegetables" , "allocation" .)