Gloomy old Iberian river (5)
I believe the answer is:
douro
'iberian river' is the definition.
(I've seen this before)
'gloomy old' is the wordplay.
'gloomy' becomes 'dour' (I've seen this before).
'old' becomes 'o' (abbreviation).
'dour'+'o'='DOURO'
(Other definitions for douro that I've seen before include "European runner" , "Iberian river" , "Hollow bubbling sound" .)