President Lincoln's tip for second son? (4)
I believe the answer is:
abel
'second son?' is the definition.
(second son of Adam and Eve)
'president lincoln's tip' is the wordplay.
'president' becomes 'Abe' (Abraham (Abe) Lincoln).
'tip' indicates taking the first letters (I've seen 'tips' mean this (tip can refer to the top end of something)).
The first letter of 'lincolns' is 'l'.
'abe'+'l'='ABEL'
'for' is the link.
(Other definitions for abel that I've seen before include "Son of Adam and Eve" , "Brother killed by Cain" , "First shepherd" , "Cain slew him" , "Eve's son" .)