Decline old pudding (4)
I believe the answer is:
sago
'pudding' is the definition.
(I've seen this before)
'decline old' is the wordplay.
'decline' becomes 'sag' (I've seen this before).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'sag'+'o'='SAGO'
(Other definitions for sago that I've seen before include "Cereal, used in pudding" , "Starchy grain used in desserts" , "dessert?" , "Starchy cereal used in puddings" , "Powdery starch used in cooking" .)