Taurus and Leo overlapping? That’s great fortune, almost certainly (7)
I believe the answer is:
bullion
'great fortune almost certainly' is the definition.
(bullion is gold, so is worth a fortune)
'taurus and leo overlapping?' is the wordplay.
'taurus' becomes 'bull' (I've seen this before).
'and' says to put letters next to each other.
'leo overlapping?' becomes 'ion' (I can't justify this - if you can you should believe this answer much more).
'bull'+'ion'='BULLION'
'that's' acts as a link.
(Other definitions for bullion that I've seen before include "Gold/silver in bulk" , "expensive bars" , "Precious metal in mass" , "Gold or silver in bars or ingots" , "Gold or silver in bulk" .)