Developing polythene bead is unacceptable (6,3,4)
I believe the answer is:
beyond the pale
'unacceptable' is the definition.
(I've seen this before)
'developing polythene bead' is the wordplay.
'developing' is an anagram indicator.
'polythene'+'bead'='polythenebead'
'polythenebead' with letters rearranged gives 'BEYOND THE PALE'.
'is' acts as a link.
(Other definitions for beyond the pale that I've seen before include "Only understood by tanned people?" , "Socially unacceptable" , "Intolerable, unacceptable" , "completely washed out?" , "Not understood by untanned people" .)