Certain marriage involves search leading to fit (14)
I believe the answer is:
unquestionable
'certain' is the definition.
(thesaurus)
'marriage involves search leading to fit' is the wordplay.
'marriage' becomes 'union' ('union' can be a synonym of 'marriage').
'involves' means one lot of letters goes inside another.
'search leading' becomes 'quest' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to fit' becomes 'able' ('able' can be similar in meaning to 'fit').
'union' placed around 'quest' is 'unquestion'.
'unquestion'+'able'='UNQUESTIONABLE'
(Another definition for unquestionable that I've seen is " beyond doubt".)