Key on completion (6)
![Ross](/clue/static/ross-mug.min.png)
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" .)