Trivial function to swap leading parts in element of theatre design (9)

'trivial function to swap leading parts' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'in element of theatre design' is the wordplay.
I cannot quite see how this works, but
'in' is within the answer.
'element' could be 'tl' (Tl is an example) and 'tl' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?