Be an accomplice in Lincoln, ending in court (4)
I believe the answer is:
abet
'be an accomplice' is the definition.
(I've seen this in another clue)
'lincoln ending in court' is the wordplay.
'lincoln' becomes 'abe' (Abraham Lincoln).
'ending in' suggests the final letters.
The last letter of 'court' is 't'.
'abe'+'t'='ABET'
'in' acts as a link.
(Other definitions for abet that I've seen before include "Assist criminally" , "Aid and assist" , "Assist in some nefarious activity" , "Incite, help" , "Help to do something wrong" .)