Virtually to slam about (6)
I believe the answer is:
almost
'virtually' is the definition.
('almost' can be a synonym of 'virtually')
'to slam about' is the wordplay.
'about' is an anagram indicator.
'to'+'slam'='toslam'
'toslam' anagrammed gives 'ALMOST'.
(Other definitions for almost that I've seen before include "As good as" , "Nearly, not quite" , "Slightly short of" , "Virtually" , "Not entirely" .)