Girl served up popular fizzy drink (6)
I believe the answer is:
nicola
'girl' is the definition.
'up popular fizzy drink' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'popular' becomes 'in'.
'fizzy drink' becomes 'cola' (I've seen this before).
'in' backwards is 'ni'.
'ni'+'cola'='NICOLA'
'served' is the link.
(Other definitions for nicola that I've seen before include "Girl" , "Girl's name" .)