Should be an expression of disgust written in the Old Testament (5)
I believe the answer is:
ought
'should be' is the definition.
(I've seen this before)
'an expression of disgust written in the old testament' is the wordplay.
'an expression of disgust' becomes 'ugh' (I've seen this in other clues).
'written in' indicates putting letters inside.
'the old testament' becomes 'ot'.
'ugh' placed within 'ot' is 'OUGHT'.
(Other definitions for ought that I've seen before include "outdated anything at all?" , "It expresses duty or obligation" , "Be logically necessary" , "Verb implying duty; zero, nothing (archaic)" , "Strange thing" .)