Lord Emsworth originally entering came first (3)
I believe the answer is:
led
'came first' is the definition.
(I've seen this before)
'lord emsworth originally entering' is the wordplay.
'lord' becomes 'ld' (abbreviation for the aristocratic title).
'originally' indicates taking the first letters.
'entering' means one lot of letters goes inside another.
The initial letter of 'emsworth' is 'e'.
'ld' placed around 'e' is 'LED'.
(Other definitions for led that I've seen before include "Guided; small light" , "had the edge" , "Went in advance of others" , "Paved the way" , "induced to" .)