Single woman in charge of keeping books? That’s ridiculous (7)
I believe the answer is:
idiotic
'ridiculous' is the definition.
(I've seen this in another clue)
'single woman in charge of keeping books?' is the wordplay.
'single' becomes 'I' (one).
'woman' becomes 'Di' (short for Diana).
'in charge of' becomes 'ic'.
'keeping' indicates putting letters inside (some letters kept inside).
'books?' becomes 'ot' (abbreviation for Old Testament).
'di'+'ic'='diic'
'diic' placed around 'ot' is 'diotic'.
'i'+'diotic'='IDIOTIC'
'that's' acts as a link.
(Other definitions for idiotic that I've seen before include "Stupid, foolish" , "Ridiculous" , "Half witted" , "Altogether nonsensical" , "Absurd, crackbrained" .)