Pith helmet, best quality one (4)
I believe the answer is:
topi
'pith helmet' is the definition.
(synonyms)
'best quality one' is the wordplay.
'best quality' becomes 'top' (top can mean of the best quality).
'one' becomes 'i' (Roman numeral).
'top'+'i'='TOPI'
(Other definitions for topi that I've seen before include "Sun protection" , "Headgear" , "Tropical sun-hat" , "Indian sun hat made with pith from sola plants" , "cover in the heat" .)