Vessel harbouring is from east state (5)
I believe the answer is:
posit
'state' is the definition.
(I know that state can be written as posit)
'vessel harbouring is from east' is the wordplay.
'vessel' becomes 'pot' (type of container).
'harbouring' means one lot of letters goes inside another.
'from east' says the letters should be written backwards (letters written right to left or 'east to west').
'is' reversed gives 'si'.
'pot' placed around 'si' is 'POSIT'.
(Other definitions for posit that I've seen before include "Lay down as principle" , "Assume something as a basis for discussion" , "Put forward as true" , "Put forward (a theory)" , "Assume as fact for argument's sake" .)