Girl facing river and extension to building (6)
I believe the answer is:
annexe
'extension to building' is the definition.
(I've seen this before)
'girl facing river' is the wordplay.
'girl' becomes 'ann'.
'facing' says to put letters next to each other.
'river' becomes 'exe' (I've seen this before).
'ann'+'exe'='ANNEXE'
'and' is the link.
(Other definitions for annexe that I've seen before include "Added section of document" , "bit on the side?" , "Supplementary part" , "Extension to main building" , "Extra wing (on building)" .)