That glen drawn extensively (2,6)
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" .)