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' means one lot of letters go next to another (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 centre of 'today' is 'd'.
'par'+'d'='pard'
'on' 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" .)