Stolid runner confronted (2-5)
I believe the answer is:
po-faced
'stolid' is the definition.
(I've seen this before)
'runner confronted' is the wordplay.
'runner' becomes 'Po' (Italian river).
'confronted' becomes 'faced' ('face' can be a synonym of 'confront').
'po'+'faced'='PO-FACED'
(Other definitions for po-faced that I've seen before include "With a disapproving countenance" , "Glum" , "Humourlessly disapproving" , "Humourless and disapproving" , "Not showing amusement, having solemn expression" .)