Revolutionary song about German capital (4)
I believe the answer is:
riga
'capital' is the definition.
(capital of Latvia)
'revolutionary song about german' is the wordplay.
'revolutionary' shows that the letters should be reversed in order.
'song' becomes 'air' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'german' becomes 'g' (abbreviation).
'air' enclosing 'g' is 'agir'.
'agir' written backwards gives 'RIGA'.
(Other definitions for riga that I've seen before include "Fix a rig up here for the Latvians" , "capital in Europe" , "A rig in Latvia" , "Capital city of Latvia" , "Baltic port" .)