Wise guy mouths "round one” (5)
I believe the answer is:
swami
'wise guy' is the definition.
(I've seen this in another clue)
'mouths round one' is the wordplay.
'mouths' becomes 'maws' (maw is a kind of mouth).
'round' says the letters should be written backwards.
'one' becomes 'i' (Roman numeral).
'maws' back-to-front is 'swam'.
'swam'+'i'='SWAMI'
(Other definitions for swami that I've seen before include "Hindu religious instructor" , "Guru" , "Hindu mystic" , "Hindu holy man" , "Hindu leader" .)