Writer, one in ball, pirouetting (4)
I believe the answer is:
biro
'writer' is the definition.
(I've seen this before)
'one in ball pirouetting' is the wordplay.
'one' becomes 'I' (Roman numeral).
'in' indicates putting letters inside.
'ball' becomes 'orb' (synonyms).
'pirouetting' shows that the letters should be reversed in order (pirouette is a spin in ballet - letters spin round).
'i' put within 'orb' is 'orib'.
'orib' back-to-front is 'BIRO'.
(Other definitions for biro that I've seen before include "Lazlo -, ballpoint pen inventor" , "Laszlo Jozsef -, ballpoint pen inventor" , "writer" , "Writing implement" , "Pocket item" .)