Occasional large-scale arrests as detective (8)
I believe the answer is:
episodic
'occasional' is the definition.
(I know that occasional can be written as episodic)
'large-scale arrests as detective' is the wordplay.
'large-scale' becomes 'epic' (I've seen this before).
'arrests' is an insertion indicator (inserted letters are held or arrested).
'as' becomes 'so' (I've seen this in other clues).
'detective' becomes 'di' (Detective Inspector abbreviation).
'so'+'di'='sodi'
'epic' enclosing 'sodi' is 'EPISODIC'.
(Other definitions for episodic that I've seen before include "Serialised" , "in serial form" , "Occurring at irregular intervals" , "taken one piece at a time" , "Presented in sections" .)