Initially, oddball adhered to his solemn vows (5)
I believe the answer is:
oaths
'vows' is the definition.
('oath' can be a synonym of 'vow')
'initially oddball adhered to his solemn' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'oddball adhered to his solemn' is 'OATHS'.
(Other definitions for oaths that I've seen before include "Cursing" , "Solemn sworn promises" , "Profane expressions" , "Curses - words of honour?" , "four-letter words" .)