Don't be so demanding - it's unnecessary (8)
I believe the answer is:
needless
'unnecessary' is the definition.
(similar in meaning)
'don't be so demanding it's' is the wordplay.
I cannot really see how this works, but
'be' could be 'need' (needing is a kind of being) and 'need' is found in the answer.
'it's' could be 'es' ('e' can mean 'electronic' which is similar to 'IT') and 'es' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for needless that I've seen before include "Without reason" , "Unneccessary and unwarranted" , "Uncalled-for" , "- - to say" , "Unnecessary and unwarranted" .)