Popular passage, one acceptable in investiture (9)
I believe the answer is:
induction
'investiture' is the definition.
(thesaurus)
'popular passage one acceptable' is the wordplay.
'popular' becomes 'in'.
'passage' becomes 'duct' (duct is a kind of passage**).
'one' becomes 'i' (Roman numeral).
'acceptable' becomes 'on' (I've seen this in other clues).
'in'+'duct'+'i'+'on'='INDUCTION'
'in' is the link.
(Other definitions for induction that I've seen before include "Logical procedure" , "advances prospects of New Labour?" , "- Loop; - heating; - coil" , "Process of reasoning" , "sort of motor" .)