Spanish runner’s gold medal, initially missing, turned up (4)
I believe the answer is:
ebro
'spanish runner's' is the definition.
(Spanish river - a river is something that runs)
'gold medal initially missing turned up' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'medal' becomes 'OBE' (Order of the British Empire).
'initially missing' means to remove the first letter.
'turned up' is a reversal indicator.
'obe' with its first letter removed is 'be'.
'or'+'be'='orbe'
'orbe' back-to-front is 'EBRO'.
(Other definitions for ebro that I've seen before include "River of northeast Spain" , "Spanish river" , "European runner" , "Iberian river" , "foreign river" .)