I make mistake filling cup for clown (7)
I believe the answer is:
pierrot
'clown' is the definition.
(I've seen this before)
'i make mistake filling cup' is the wordplay.
'make mistake' becomes 'err' (I've seen this before).
'filling' is an insertion indicator.
'cup' becomes 'pot'.
'i'+'err'='ierr'
'ierr' going inside 'pot' is 'PIERROT'.
'for' acts as a link.
(Other definitions for pierrot that I've seen before include "In French panto, clown with white face and baggy costume" , "Type of clown" , "Aged entertainer" , "French pantomime character with a white face" , "Period; incantation" .)