A shrew or a pugnacious old person (8)

I believe the answer is:

warhorse

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'person' is the definition.
(warhorse is a kind of person)

'a shrew or a pugnacious old' is the wordplay.
'a shrew or' anagrams to 'WARHORSE'.
Though, I am not clear how this is indicated.

Can you help me to learn more?

(Other definitions for warhorse that I've seen before include "Old soldier" , "Belligerent" , "Copenhagen, say" , "steed?" , "something too often played" .)

I've seen this clue in The Irish Independent.

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