Expression of gratitude about a snap capturing ordinary pudding (7)
I believe the answer is:
tapioca
'pudding' is the definition.
(tapioca pudding is a kind of pudding)
'expression of gratitude about a snap capturing ordinary' is the wordplay.
'expression of gratitude' becomes 'ta' (informal way of saying 'thank you').
'about' indicates putting letters inside.
'snap' becomes 'pic'.
'capturing' is an insertion indicator.
'ordinary' becomes 'o' (abbreviation e.g. O-Levels).
'a'+'pic'='apic'
'apic' going around 'o' is 'apioc'.
'ta' enclosing 'apioc' is 'TAPIOCA'.
(Other definitions for tapioca that I've seen before include "Ciao, Pat, have some strange pudding" , "I act OAP (anag) - edible substance made from cassava" , "Sago pudding" , "it thickens" , "dessert" .)