A gust swirling, going through Lisbon (5)
I believe the answer is:
tagus
'going through lisbon' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'a gust swirling' is the wordplay.
'swirling' indicates anagramming the letters (letters get swirled around).
'a'+'gust'='agust'
'agust' is an anagram of 'TAGUS'.
Can you help me to learn more?
(Other definitions for tagus that I've seen before include "Portuguese river flowing to sea near Lisbon" , "Longest river of Iberian peninsular" , "Current" , "Longest river of Iberian peninsula" , "banker" .)