Capital gets behind French speaking song (11,3)
I believe the answer is:
londonderry air
'song' is the definition.
(I've seen this in another clue)
'capital gets behind french speaking' is the wordplay.
I cannot quite understand how this works, but
'capital' could be 'london' (capital of the United Kingdom) and 'london' is present in the answer.
This explanation may well be incorrect...
(Other definitions for londonderry air that I've seen before include "anthem of N Ireland?" , "traditional composition" .)