Girl's fortieth (4)
I believe the answer is:
ruby
I believe this is a double definition.
'girl's' is the first definition.
(girl's name)
'fortieth' is the second definition.
(a ruby anniversary marks 40 years)
(Other definitions for ruby that I've seen before include "Deep red - transparent gemstone" , "Gemstone comprising mainly aluminium oxide" , "A red jewel" , "Deep 6 across" , "Completely" .)