To Greeks at first a Roman habit (4)
I believe the answer is:
toga
'a roman habit' is the definition.
(garment worn in ancient Rome)
'to greeks at first' is the wordplay.
'first' suggests taking the first letters.
The first letters of 'greeks at' is 'ga'.
'to'+'ga'='TOGA'
(Other definitions for toga that I've seen before include "piece of old clothing" , "Classical garment" , "Garment worn by men in ancient Rome" , "Roman dress" , "Graph's y value" .)