15 across
Chief
competitor,
if
losing
run,
would
be
matter
of
record
(4,5)
Chief competitor, if losing run, would be matter of record (4,5)
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The definition is at the beginning of the clue.
How the clue works
Right, so Chief competitor is the definition, pointing us to an arch rival. Now the wordplay's a bit of a game: the clue says if you take this answer and lose an R (that's "run" being cricket shorthand as usual), you're left with ARCHIVAL, which means "matter of record" — and sure enough, whip the R out of ARCH RIVAL and that's exactly what you get.