That happy ending ahead (3)

I believe the answer is:

yon

'that' is the definition.
(archaic term meaning 'that')

'happy ending ahead' is the wordplay.
'ending' says to take the final letters (the letter at the end of the word).
'ahead' becomes 'on' (synonyms).
The final letter of 'happy' is 'y'.
'y'+'on'='YON'

(Another definition for yon that I've seen is " Those Scots".)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app