That glen is to collapse eventually (2,6)
I believe the answer is:
at length
'eventually' is the definition.
('at length' can be a synonym of 'eventually')
'that glen is to collapse' is the wordplay.
'is to collapse' indicates an anagram.
'that'+'glen'='thatglen'
'thatglen' with letters rearranged gives 'AT LENGTH'.
(Other definitions for at length that I've seen before include "Exhaustively" , "In exhaustive detail, after a long time" , "eventually" , "In due course" , "Fully extended" .)