Following stream is an unnecessary extra (5)
I believe the answer is:
frill
'an unnecessary extra' is the definition.
(I've seen this in another clue)
'following stream' is the wordplay.
'following' becomes 'f' (used when citing page numbers etc).
'stream' becomes 'rill' (rill is a kind of stream).
'f'+'rill'='FRILL'
'is' acts as a link.
(Other definitions for frill that I've seen before include "Gathered strip as a trimming" , "Unnecessary embellishment" , "Extra embellishment" , "Pleated strip of cloth trimming or an unnecessary extra" , "An unnecessary extra or ornamental strip" .)