Tackle occasional roguery with severity (6)
I believe the answer is:
rigour
'severity' is the definition.
(I know that severity can be written as rigour)
'tackle occasional roguery' is the wordplay.
'tackle' becomes 'rig' (I've seen this in other clues).
'occasional' means one should take alternating letters (letters taken at regular intervals or on occasion).
The alternating letters of 'roguery' are 'our'.
'rig'+'our'='RIGOUR'
'with' acts as a link.
(Other definitions for rigour that I've seen before include "Severity or stringency" , "Severity or strictness, perhaps of the law" , "Preciseness - severity" , "Trial" , "Extreme care and thoroughness" .)