Wham!’s crazy old instrument (5)
I believe the answer is:
shawm
'old instrument' is the definition.
(historical type of wind instrument)
'wham's crazy' is the wordplay.
'crazy' indicates anagramming the letters.
'whams' anagrammed gives 'SHAWM'.
(Other definitions for shawm that I've seen before include "Medieval and Renaissance wind instrument" , "Old wind instrument" , "old music-maker" .)