No problem replacing this formal acknowledgement of gratitude (5,7)
I believe the answer is:
youre welcome
'no' is the definition.
Both the definition and answer are adjectives. Maybe there's a link between them I don't understand?
'problem replacing this formal acknowledgement of gratitude' is the wordplay.
I cannot quite see how this works, but
'acknowledgement' could be 'welcome' (welcome is a kind of acknowledgement) and 'welcome' is found in the answer.
'of' could be 'o' and 'o' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for youre welcome that I've seen before include "It was no trouble" , "My pleasure" , "Polite response to thanks" , "Don't mention it" , "No problem!" .)