Gem in a ring given to a friend (4)
I believe the answer is:
opal
'gem' is the definition.
(I know that opal is a type of opaque gem)
'ring given to a friend' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'given to' means one lot of letters go next to another.
'a friend' becomes 'pal' (pal can mean a friend).
'o'+'pal'='OPAL'
'in a' is the link.
(Other definitions for opal that I've seen before include "Type of gemstone" , "precious jewel" , "Milky white gemstone" , "Many-coloured gem" , "what may be set in 29 [DIAL]" .)