One first in but rarely last out! (6)
I believe the answer is:
opener
'one first' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for opener that I've seen before include "First going in to bat" , "cricketer receiving the first ball?" , "Introductory remark" , "Card player who starts the bidding" , "Batsman who starts innings" .)