Marry after extremely regrettable service (6)
I believe the answer is:
repair
'service' is the definition.
(I've seen this before)
'marry after extremely regrettable' is the wordplay.
'marry' becomes 'pair' (I can't justify this - if you can you should believe this answer much more).
'after' means one lot of letters go next to another.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'regrettable' with its centre removed is 're'.
'pair' after 're' is 'REPAIR'.
Can you help me to learn more?
(Other definitions for repair that I've seen before include "Restore to good order" , "Proceed - restore" , "resort" , "Get back into good shape" , "Travel or proceed toward some place" .)