This person is underlining a point (3)
I believe the answer is:
aim
'point' is the definition.
(I've seen this before)
'this person is underlining a' is the wordplay.
'this person is' becomes 'im' ('this person' can refer to the person speaking).
'underlining' means one lot of letters go next to another (in a down clue, some letters go underneath others).
'im' put after 'a' is 'AIM'.
(Other definitions for aim that I've seen before include "Plan" , "Try to achieve" , "Set sights on, target or goal" , "This is mean" , "mission" .)