I'm dividing assets: that's rough guess (8)
I believe the answer is:
estimate
'rough guess' is the definition.
(I've seen this before)
'i'm dividing assets' is the wordplay.
'dividing' is an insertion indicator (inserted letters divide the word).
'assets' becomes 'estate' (I've seen this in another clue).
'im' inserted within 'estate' is 'ESTIMATE'.
'that's' acts as a link.
(Other definitions for estimate that I've seen before include "Statement of expected cost" , "Tease Tim (anag.)" , "Guess the cost" , "Judge the worth of" , "damage" .)