Show regret overtaking Tesla, say (5)
I believe the answer is:
revue
'show' is the definition.
(revue is a kind of show)
'regret overtaking tesla say' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'overtaking' is an insertion indicator (a word 'takes over' another set of letters).
'tesla say' becomes 'ev' (I can't explain this - if you can you should believe this answer much more).
'rue' going around 'ev' is 'REVUE'.
Can you help me to learn more?
(Other definitions for revue that I've seen before include "Show of sketches, songs, etc" , "Form of light entertainment" , "Light theatrical entertainment" , "Entertainment consisting of sketches" , "Theatrical show" .)