Way of working, possibly pseudorandom one (5,8)
I believe the answer is:
modus operandi
'way of working' is the definition.
(someone's usual way of working)
'possibly pseudorandom one' is the wordplay.
'possibly' indicates anagramming the letters.
'one' becomes 'I' (Roman numeral).
'pseudorandom' is an anagram of 'modusoperand'.
'modusoperand'+'i'='MODUS OPERANDI'
(Other definitions for modus operandi that I've seen before include "this shows how it's done" , "Method of working" , "Particular way of doing something (Latin)" , "Classically typical style" , "way of doing things" .)