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' says to put letters next to each other.
'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" .)