It follows forthwith — usually — but not all of it (4)
I believe the answer is:
thus
'it follows' is the definition.
(both can mean 'therefore')
'forthwith usually but not all of it' is the wordplay.
'not all of it' indicates a hidden word (only some of the letters are needed).
'THUS' is hidden in the letters of 'forthwith usually but'.
(Other definitions for thus that I've seen before include "In this manner" , "Like so" , "an original Christmas gift" , "As a result" , "Therefore, hence" .)