Tango unknown in country dances (7)
I believe the answer is:
waltzes
'dances' is the definition.
(waltz is a kind of dance)
'tango unknown in country' is the wordplay.
'tango' becomes 'T' (phonetic alphabet: alpha, bravo, charlie etc.).
'unknown' becomes 'z' (represents an unknown value in algebra).
'in' indicates putting letters inside.
'country' becomes 'wales' (country of the UK).
't'+'z'='tz'
'tz' put inside 'wales' is 'WALTZES'.
(Other definitions for waltzes that I've seen before include "Dances" , "Musical compositions for dancing a la Strauss" , "Strauss wrote some classic dance melodies" .)