Some points on idiomatic Clojure:

• Do not return nil from if statements: (if condition? nil false-branch) -> (if-not condition? false-branch)
• defn bodies in the next line unless they are truly one liners (not like in Booth-Booked-Indicator.
• (eval f) -> (f).

Conceptually:

• If you want to define your own types use defrecord and deftype (defrecord in this case). Poss-Dupe, Order-Time Order-Date, etc. should be records, and it might be cleaner if they use the same fn bindings, instead of [[x]] and [x], only one of the two.
• Instead of defining of passing around a function, evaluating it, checking for a NullPointerException, and then going to the default implementation, consider using a protocol. For example, IParse (parse [f]). You can then implement parse for your different types and leave a default implementation. Then try-call will be just reduced to -parse.

Some points on idiomatic Clojure:

• Do not return nil from if statements: (if condition? nil false-branch) -> (if-not condition? false-branch)
• defn bodies in the next line unless they are truly one liners (not like in Booth-Booked-Indicator.
• Don't use eval but resolve the function and call it directly as (f).

Conceptually:

• If you want to define your own types use defrecord and deftype (defrecord in this case). Poss-Dupe, Order-Time Order-Date, etc. should be records, and it might be cleaner if they use the same fn bindings, instead of [[x]] and [x], only one of the two.
• Instead of defining of passing around a function, evaluating it, checking for a NullPointerException, and then going to the default implementation, consider using a protocol. For example, IParse (parse [f]). You can then implement parse for your different types and leave a default implementation. Then try-call will be just reduced to -parse.