Promise of ketchup at last appearing on shelf (6)
I believe the answer is:
pledge
'promise' is the definition.
(pledging is a kind of promising)
'ketchup at last appearing on shelf' is the wordplay.
'at last' suggests the final letters.
'appearing on' is a charade indicator (letters next to each other).
'shelf' becomes 'ledge' (synonyms).
The last letter of 'ketchup' is 'p'.
'p'+'ledge'='PLEDGE'
'of' acts as a link.
(Other definitions for pledge that I've seen before include "Promise, word of honour" , "Promise solemnly and formally" , "Vow; security" , "Firm commitment" , "Promise (to abstain?)" .)