Since amongst English, Yankee's relaxed (4)
I believe the answer is:
easy
'relaxed' is the definition.
(associated in meaning)
'since amongst english yankee's' is the wordplay.
'since' becomes 'as' (both can mean 'because').
'amongst' means one lot of letters goes inside another (I've seen this in other clues).
'english' becomes 'E' (abbreviation).
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'e'+'y'='ey'
'as' put into 'ey' is 'EASY'.
(Other definitions for easy that I've seen before include "Promiscuous" , "Gentle - light" , "Effortless, uncomplicated" , "Simply achieved" , "Not difficult" .)