Musical growth (4)
I believe the answer is:
hair
This clue is a double definition.
'musical' is the first definition.
(1960s musical)
'growth' is the second definition.
(I've seen this before)
(Other definitions for hair that I've seen before include "A very small quantity" , "It helps prevent heat loss through the head" , "Famous hirsute musical with song ` `Aquarius' ' (4)" , "Growth from skin" , "Musical with nude scenes" .)