"May I help you?" (3)
I believe the answer is:
yes
'may i help you?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yes that I've seen before include "Of course" , "Yea" , "One response to "will you marry me?"" , "Positive response" , "That's 27 [RIGHT]" .)