Girl taking a long time to produce a piece of mosaic (7)
I believe the answer is:
tessera
'a piece of mosaic' is the definition.
(I've seen this before)
'girl taking a long time' is the wordplay.
'girl' becomes 'tess' (girl's name).
'taking' means one lot of letters go next to another.
'a long time' becomes 'era' (a long period of time).
'tess'+'era'='TESSERA'
'to produce' is the link.
(Other definitions for tessera that I've seen before include "Small square tile used in mosaics" , "Small mosaic piece" , "small bit of artwork" , "Reseats (anag) -- small tile used in a mosaic" , "part of a mosaic" .)