Small piece on Sydney's top pin-up (4)
I believe the answer is:
snip
'small piece' is the definition.
(I've seen this in another clue)
'sydney's top pin-up' is the wordplay.
'top' says to take the initial letters.
'up' says the letters should be written backwards (in down clue: letters go upwards).
The initial letter of 'sydney' is 's'.
'pin' backwards is 'nip'.
's'+'nip'='SNIP'
'on' is the link.
(Other definitions for snip that I've seen before include "nick)" , "Cut; bargain (colloq.)" , "Cut with shears" , "Cut with scissors in small strokes" , "Bargain - crop" .)