On completion produces key (6)
I believe the answer is:
legend
'key' is the definition.
(thesaurus)
'on completion' is the wordplay.
'on' becomes 'leg' (cricket term, fielding positions).
'completion' becomes 'end' (I've seen this before).
'leg'+'end'='LEGEND'
'produces' acts as a link.
(Other definitions for legend that I've seen before include "Very famous member's death" , "Stanley Matthews, perhaps" , "Inscription; myth" , "explanation" , "Old tale" .)