What’s on is below standard today essentially (6)
I believe the answer is:
pardon
'what's' is the definition.
(I've seen this before)
'on is below standard today essentially' is the wordplay.
'is below' says to put letters next to each other (in a down clue, some letters go below others).
'standard' becomes 'par' (standard of play in golf).
'essentially' means to look at the middle letters.
The middle letter of 'today' is 'd'.
'par'+'d'='pard'
'on' put after 'pard' is 'PARDON'.
(Other definitions for pardon that I've seen before include "Forgive, perhaps judicially" , "Forgive; come again?" , "Excuse or absolve" , "indulgence" , "What was that you said" .)