Love friend with gem (4)
I believe the answer is:
opal
'gem' is the definition.
(I know that opal is a type of opaque gem)
'love friend' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'friend' becomes 'pal' (pal can mean a friend).
'o'+'pal'='OPAL'
'with' is the link.
(Other definitions for opal that I've seen before include "Milky white gem" , "Translucent gem" , "Milky-white mineral" , "Many-coloured gem" , "Birthstone for October" .)