To the Yankee, a mere plaything (3)
I believe the answer is:
toy
'a mere plaything' is the definition.
'toy' can be an answer for 'plaything' (synonyms). I am not sure about the 'a mere' bit.
'to the yankee' is the wordplay.
'the yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'to'+'y'='TOY'
Can you help me to learn more?
(Other definitions for toy that I've seen before include "Teddy or Dolly" , "Trifle or dally with" , "Doll or teddy, say" , "Trifle with plaything?" , "One of spelt out words" .)