What dreams were for Hamlet, with mad hero about to act decisively (5,3,7)
I believe the answer is:
cross the rubicon
'to act decisively' is the definition.
(reference to Caesar crossing the Rubicon river)
'what dreams were for hamlet with mad hero' is the wordplay.
'what dreams were for hamlet' becomes 'therub' (Hamlet says 'ay, there's the rub' about dreams).
'with' means one lot of letters go next to another.
'mad' becomes 'cross' (both can mean angry).
'hero' becomes 'icon'.
'therub' put after 'cross' is 'crosstherub'.
'crosstherub'+'icon'='CROSS THE RUBICON'
'about' is the link.
(Other definitions for cross the rubicon that I've seen before include "make a commitment" , "Go over border river" , "take fateful step" , "to commit oneself?" , "burn bridges" .)