Key: one of two in first (6)
I believe the answer is:
opener
I believe this is a triple definition!
'key' is the first definition.
(I've seen this before)
'one of two' is the second definition.
(cricket term: one of two opening batsmen)
'in' is a link.
'first' is the third definition.
(I've seen this before)
(Other definitions for opener that I've seen before include "See 1" , "First goal in game" , "batsman facing the first ball?" , "Initial event" , "initial goal" .)