I've made a sauce with some white wine (with cream & tarragon) but it's really sharp (prob the wine was a bit old, or I didn't boil it off well enough - is there any way to fix it? and what's the most likely cause?
If "sharp" means too much alcohol left, bring it back to a simmer and cook off more of it, then adjust with more cream if needed.
If "sharp" means too acidic, your options are:
(1) add more cream and other ingredients to dilute the acid (2) try to mask the acid with a bit of sugar (3) I guess you could try to neutralize the acid with a very small amount of something basic, like baking soda. But that seems like a bad idea, it will probably taste terrible.