It’s currently not possible to add conditions to a Deep Clone post function. However, you can achieve the same effect by attaching a condition to the workflow transition that contains the Deep Clone post function and have another transition without the Deep Clone post function.
Workflow Transition 1:
Has condition “Field has value”
Has post-function for Deep Clone
Workflow Transition 2:
Has condition “Field does not have value”
Doesn’t have post-function for Deep Clone
You can set up your workflow to show either one or the other. Unfortunately the transitions cannot have the same name.