Pied Piper's followers (4)
I believe the answer is:
rats
'pied piper's followers' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rats that I've seen before include "were piped away" , "First word of 'The 21 19'" , "Tells" , "Heels" , "Mild oath - sewer residents" .)