As usual, “round about one” means two (4)
I believe the answer is:
pair
'two' is the definition.
(pair is a kind of two)
'as usual round about one' is the wordplay.
'as usual round' becomes 'par' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'par' placed around 'i' is 'PAIR'.
'means' acts as a link.
Can you help me to learn more?
(Other definitions for pair that I've seen before include "Put together - two of a kind" , "Cereal used as food" , "flight of stairs" , "players of duet?" , "E.g. two aces; mate" .)