18 across
Perhaps
one
in
stock
here
if
wanting
order
(6)
Perhaps one in stock here if wanting order (6)
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The definition is at the beginning of the clue.
How the clue works
Have a look at Perhaps one in stock — that's your definition, since a heifer is just the kind of beast you'd find kept as livestock. For the wordplay, take "here if" and give it a shuffle, since "wanting order" is telling you it's an anagram — jumble those letters and out comes HEIFER.