Style of column — one round and mostly attractive (5)
I believe the answer is:
ionic
'style of column' is the definition.
(I've seen this in another clue)
'one round and mostly attractive' is the wordplay.
'one' becomes 'i' (Roman numeral).
'round' becomes 'o' (round can mean a round object).
'and' says to put letters next to each other.
'mostly' means to remove the last letter (most but not all of the word).
'attractive' becomes 'nice' (nice can mean attractive).
'nice' with its final letter taken away is 'nic'.
'i'+'o'+'nic'='IONIC'
(Other definitions for ionic that I've seen before include "A classical order of architecture" , "Greek school" , "Order for architects" , "Description of bond" , "Style of Greek column used for the British Museum and the US Capitol" .)