Originally Posted by NewHope
were it left to me - This is normally "if it were left to me". It's possible to begin a second or third conditional sentence with "were". This could be considered an inversion. It's more formal. It's not something that is used often in English, but it is used. It has a more serious tone.
were + noun or noun phrase = if + noun or noun phrase + were
were + it = if it were
In your example sentence "pin down" means "define". I take "pin down" to mean "define". As Cambridge Dictionaries Online points out, "pin down" can also mean "discover exact details about something". I suppose if we discover exact details about something, then we can also say that we discover the definition of something.
Dictionary.com has this to say about "pin down":
To fix or establish clearly: was finally able to pin down the cause of the disease.
Fix or establish clearly, as in The firefighters finally were able to pin down the source of the odor. [Mid-1900s]
Here, it would appear that Dictionary.com understands "pin down" as I do in your example sentence.
v 1: define clearly; "I cannot narrow down the rules for this game"