Pith helmet, best one (4)
I believe the answer is:
topi
'pith helmet' is the definition.
('topi' can be a synonym of 'pith helmet')
'best one' is the wordplay.
'best' becomes 'top' ('top' can be similar in meaning to 'best').
'one' becomes 'i' (Roman numeral).
'top'+'i'='TOPI'
(Other definitions for topi that I've seen before include "Pith-helmet" , "Tropical sun-hat" , "Large antelope; pith hat" , "Pith helmet" , "Item of clothing" .)