Take it before total collapse, finally (7)
I believe the answer is:
presume
'take it' is the definition.
(I've seen this before)
'before total collapse finally' is the wordplay.
'before' becomes 'pre'.
'total' becomes 'sum' ('sum' can be a synonym of 'total').
'finally' indicates one should take the final letters.
The last letter of 'collapse' is 'e'.
'pre'+'sum'+'e'='PRESUME'
(Other definitions for presume that I've seen before include "Suppose without proof" , "Suppose; take liberties" , "Take to be the case" , "Believe" , "Supreme (anag)" .)