One makes routine observation true for each, possibly? Somewhat (8,2,5)
I believe the answer is:
creature of habit
'one makes routine observation' is the definition.
(someone who observes a routine)
'true for each possibly? somewhat' is the wordplay.
'possibly?' indicates anagramming the letters (I've seen 'possibly' mean this).
'somewhat' becomes 'abit' (a bit).
'true'+'for'+'each'='trueforeach'
'trueforeach' with letters rearranged gives 'creatureofh'.
'creatureofh'+'abit'='CREATURE OF HABIT'
(Another definition for creature of habit that I've seen is " one who's predictable?".)