Keen on country, in a manner of speaking (10)
I believe the answer is:
intonation
'manner of speaking' is the definition.
(intonation is a kind of manner of speaking)
'keen on country' is the wordplay.
'keen on' becomes 'into' (if you're 'into' something you're keen on it).
'country' becomes 'nation' (synonyms).
'into'+'nation'='INTONATION'
'in a' acts as a link.
(Other definitions for intonation that I've seen before include "Feature of speech" , "Rise and fall in voice pitch" , "bit of plainsong" .)