One might say this very much as an expression of gratefulness
I believe the answer is:
thank you
'expression of gratefulness' is the definition.
'thank you' can be an answer for 'expression' (I have seen 'Expression of gratitude' mean 'thank you' so perhaps 'expression' could also mean 'thank you'). I am unsure of the 'of gratefulness' bit.
'one might say this very much as an' is the wordplay.
I cannot really see how this works, but
'one' could be 'an' (I've seen this in other clues) and 'an' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for thank you that I've seen before include "Conventional expression of gratitude" , "I'm grateful" , "Words of gratitude" , "Much obliged!" , "Polite reply (to gift)" .)