Sign that a group's forgotten tango dancing (9)
I believe the answer is:
autograph
'sign' is the definition.
(autographing is a kind of signing)
'that a group's forgotten tango dancing' is the wordplay.
'forgotten' is a deletion indicator.
'tango' becomes 't' (phonetic alphabet: alpha, bravo, charlie etc.).
'dancing' indicates an anagram.
'that'+'a'+'group'='thatagroup'
'thatagroup' with 't' taken away is 'hatagroup'.
'hatagroup' is an anagram of 'AUTOGRAPH'.
(Other definitions for autograph that I've seen before include "Handwriting" , "One's signed" , "Famous person's signature" , "(Celebrity's) signature" , "Sign" .)