Require return of label in the middle of muddle (6)
I believe the answer is:
demand
'require' is the definition.
(I know that require can be written as demand)
'return of label in the middle of muddle' is the wordplay.
'return of' says the letters should be written backwards.
'label' becomes 'name' (naming is a kind of labelling).
'in the' indicates putting letters inside.
'middle of' says to take the centre.
The central letters of 'muddle' are 'dd'.
'name' put within 'dd' is 'dnamed'.
'dnamed' in reverse letter order is 'DEMAND'.
(Other definitions for demand that I've seen before include "Urgent or peremptory request" , "An imperious request made as if by right" , "Insistent request" , "Request made emphatically as by right" , "Requisition" .)