So-and-so, with task almost entirely messed up (7)
I believe the answer is:
whatsit
'so-and-so' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'with task almost entirely messed up' is the wordplay.
'almost entirely' means to remove the last letter (most but not all of the word).
'messed up' indicates anagramming the letters.
'with'+'task'='withtask'
'withtask' with its last letter taken off is 'withtas'.
'withtas' with letters rearranged gives 'WHATSIT'.
Can you help me to learn more?
(Other definitions for whatsit that I've seen before include "person not to be named" , "Thingumajig" , "something indefinable" , "Thingummy" , "Something one can't recall" .)