Shrewd hacker no amateur (6)
I believe the answer is:
clever
'shrewd' is the definition.
(I've seen this in another clue)
'hacker no amateur' is the wordplay.
'hacker' becomes 'cleaver' (blade for hacking at things).
'no' is a deletion indicator.
'amateur' becomes 'a'.
'cleaver' with 'a' removed is 'CLEVER'.
(Other definitions for clever that I've seen before include "Smar and canny" , "Bright; smart" , "Highly intelligent" , "Good or quick at learning" , "Sharp and able" .)