Crossword Genius

Key on completion (6)

Ross

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'

(Other definitions for legend that I've seen before include "Story from long ago" , "Footnote" , "Story that may be untrue" , "Myth or fable" , "Myth or map symbol" .)

I've seen this clue in The Telegraph.
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