Sixties musical; tresses (4)
I believe the answer is:
hair
This clue is a double definition.
'sixties musical' is the first definition.
(American musical)
'tresses' is the second definition.
(I've seen this before)
(Other definitions for hair that I've seen before include "Filaments growing on the epidermis" , "Stage musical" , "Showy garden flower" , "Barber's speciality; musical" , "Whiskers" .)