Got an unusual letter from NATO? (5)
I believe the answer is:
tango
'letter from nato?' is the definition.
(NATO phonetic alphabet)
'got an unusual' is the wordplay.
'unusual' indicates anagramming the letters.
'got'+'an'='gotan'
'gotan' anagrammed gives 'TANGO'.
(Other definitions for tango that I've seen before include "Argentinian dance form" , "Tonga (anag.)" , "Dance music" , "reputedly takes two" , "musical piece" .)