Member given object, a key (6)
I believe the answer is:
legend
'a key' is the definition.
(thesaurus)
'member given object' is the wordplay.
'member' becomes 'leg' (I've seen this before).
'given' means one lot of letters go next to another.
'object' becomes 'end' (I've seen this in other clues**).
'leg'+'end'='LEGEND'
(Other definitions for legend that I've seen before include "famous player?" , "Inscription; myth" , "Old tale" , "Person who has become famous" , "Fabulous tale" .)