Cost of crossing bridges? Going over river, and this! (5)
I believe the answer is:
troll
'this' is the definition.
The definition suggests a singular noun which matches the answer.
'cost of crossing bridges? going over river' is the wordplay.
'cost of crossing bridges?' becomes 'toll'.
'going over' indicates putting letters inside.
'river' becomes 'r'.
'toll' enclosing 'r' is 'TROLL'.
'and' is the link.
(Other definitions for troll that I've seen before include "Evil dwarf" , "Mythological being" , "Ugly Norse dwarf" , "Online pest" , "Grotesque dwarf" .)