Day inexperienced defender causes problem (8)
I believe the answer is:
drawback
'problem' is the definition.
(thesaurus)
'day inexperienced defender' is the wordplay.
'day' becomes 'd'.
'inexperienced' becomes 'raw' ('raw' can be similar in meaning to 'inexperienced').
'defender' becomes 'back' (I've seen this in other clues).
'd'+'raw'+'back'='DRAWBACK'
'causes' is the link.
(Other definitions for drawback that I've seen before include "Catch" , "Obstacle" , "Snag" , "Fly in the ointment" , "Limitation, disadvantage" .)