Building rugby posts alongside river (5)
I believe the answer is:
house
'building' is the definition.
(I've seen this before)
'rugby posts alongside river' is the wordplay.
'rugby posts' becomes 'H' (rugby goalposts resemble the letter H).
'alongside' means one lot of letters go next to another.
'river' becomes 'ouse' (Yorkshire river).
'h'+'ouse'='HOUSE'
(Other definitions for house that I've seen before include "Accommodate" , "Store" , "Residence; legislative meeting-place" , "Dwelling-place" , "Building for dwelling in" .)