Unmelodious tango, if not full of energy (8)
I believe the answer is:
tuneless
'unmelodious' is the definition.
('tuneless' can be similar in meaning to 'unmelodious')
'tango if not full of energy' is the wordplay.
'tango' becomes 't' (phonetic alphabet: alpha, bravo, charlie etc.).
'if not' becomes 'unless' (I've seen this before).
'full of' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'unless' enclosing 'e' is 'uneless'.
't'+'uneless'='TUNELESS'
(Other definitions for tuneless that I've seen before include "spoiled by lack of air?" , "Like some whistling" , "Not melodious" , "Unpleasant to hear" , "Unmelodious" .)