Alluring, singular, old partner seen with Yankee (4)
I believe the answer is:
sexy
'alluring' is the definition.
(I've seen this in another clue)
'singular old partner seen with yankee' is the wordplay.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'old partner' becomes 'ex' (an ex-partner).
'seen with' means one lot of letters go next to another.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
's'+'ex'+'y'='SEXY'
(Other definitions for sexy that I've seen before include "Titillating" , "Attractive (sl.)" , "Exciting carnal desire" , "Hot" , "arousing passion" .)