Horse that won the 1979 Epsom Derby (4)

I believe the answer is:

troy

(Other definitions for troy that I've seen before include "System of weights for precious stones and metals" , "Hardy sergeant" , "City besieged by the Greeks - system of gem weight measurement" , "System of weights for measuring precious metals and gemstones" , "Ancient city besieged to free beautiful Helen" .)

I've seen this clue in The Mirror.

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