Gain in material for building over river (9)
I believe the answer is:
increment
'gain' is the definition.
(thesaurus)
'in material for building over river' is the wordplay.
'material for building' becomes 'cement' (I can't justify this - if you can you should believe this answer much more).
'over' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'cement' placed around 'r' is 'crement'.
'in'+'crement'='INCREMENT'
Can you help me to learn more?
(Other definitions for increment that I've seen before include "Step" , "Addition to wages etc" , "Rise" , "Amount by which something increases" , "growth" .)