Old girlfriend given diamonds (5)
I believe the answer is:
dated
'old' is the definition.
(thesaurus)
'girlfriend given diamonds' is the wordplay.
'girlfriend' becomes 'date' (I've seen this in other clues).
'given' says to put letters next to each other.
'diamonds' becomes 'd' (abbreviation in bridge say).
'date'+'d'='DATED'
(Other definitions for dated that I've seen before include "Walked out with" , "Antiquated" , "Rusty" , "Archaic; took out" , "Went steady" .)