Innocent wedding vow contains one term of endearment (5)
I believe the answer is:
idiot
'innocent' is the definition.
(both can mean naive person)
'wedding vow contains one term of endearment' is the wordplay.
'wedding vow' becomes 'ido' ('I do' is part of the traditional wedding vows).
'contains' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'term of' indicates one should take the final letters (term can mean the terminus or ending of the word).
The final letter of 'endearment' is 't'.
'ido' placed around 'i' is 'idio'.
'idio'+'t'='IDIOT'
(Other definitions for idiot that I've seen before include "Fool, silly person" , "Oaf" , "Foolish character" , "nutter" , "The - (Dostoievsky)" .)