Crossword Genius

That glen drawn extensively (2,6)

Ross

I believe the answer is:

at length

'extensively' is the definition.
(thesaurus)

'that glen drawn' is the wordplay.
'drawn' indicates anagramming the letters (the letters drawn up in a different way).
'that'+'glen'='thatglen'
'thatglen' with letters rearranged gives 'AT LENGTH'.

(Other definitions for at length that I've seen before include "Finally" , "taking ages" , "In detail, taking a long time" , "Fully extended" , "Taking a long time like long-winded speech" .)

I've seen this clue in The Telegraph.
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