Oddly deficient, down to last husband? Bound to be honest (2,4)
I believe the answer is:
on oath
'bound to be honest' is the definition.
(eg under oath in a court)
'oddly deficient down to last husband?' is the wordplay.
'oddly deficient' means one should take alternating letters (letters in odd-numbered positions removed).
'husband?' becomes 'h'.
'down'+'to'+'last'='downtolast'
The alternate letters of 'downtolast' are 'onoat'.
'onoat'+'h'='ON OATH'
(Other definitions for on oath that I've seen before include "Bound to speak the truth" , "Sworn to speak the truth" .)