Runner's weight captured by electronics (4)
I believe the answer is:
elbe
'runner's' is the definition.
(river in Germany)
'weight captured by electronics' is the wordplay.
'weight' becomes 'lb' (lb is a kind of weight).
'captured by' is an insertion indicator.
'electronics' becomes 'ee' (two E's - as in E-mail).
'lb' placed into 'ee' is 'ELBE'.
(Other definitions for elbe that I've seen before include "River flowing through Dresden and Hamburg" , "River rising in the Czech Republic and flowing into the North Sea" , "Dresden/Hamburg river" , "German banker" , "River of Germany" .)