Fast tango and light dancing with unknown character (7)
I believe the answer is:
tightly
'fast' is the definition.
(fast can mean tightly or securely)
'tango and light dancing with unknown character' is the wordplay.
'tango' becomes 'T' (phonetic alphabet: alpha, bravo, charlie etc.).
'and' is a charade indicator (letters next to each other).
'dancing with' indicates anagramming the letters.
'unknown character' becomes 'y' (algebra).
'light' mixed up with 'y' gives 'ightly'.
't'+'ightly'='TIGHTLY'
(Other definitions for tightly that I've seen before include "Securely" , "In a close-fitting manner" .)