Having left later, dallies with Yankee in front of others (7)
I believe the answer is:
firstly
'in front of others' is the definition.
(first in line)
'having left later dallies with yankee' is the wordplay.
I cannot quite understand how this works, but
'left' could be 'l' (common abbreviation) and 'l' is present in the answer.
'yankee' could be 'y' (phonetic alphabet: alpha, bravo, charlie etc.) and 'y' is present in the answer.
The remaining letters 'first' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for firstly that I've seen is " Primarily".)