A grateful expression (5,3)
I believe the answer is:
thank you
'expression' is the definition.
(I have seen 'Expression of gratitude' mean 'thank you' so perhaps 'expression' could also mean 'thank you')
'a grateful' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for thank you that I've seen before include "Concentional expression of appreciation" , "Cheers" , "Words of acknowledgement" , "I appreciate that" , "Ta" .)