Problem that starts impacting some site users experience (5)
I believe the answer is:
issue
'problem' is the definition.
(issue can mean a problem or difficulty)
'starts impacting some site users experience' is the wordplay.
'starts' suggests taking the first letters.
The initial letters of 'impacting some site users experience' is 'ISSUE'.
'that' acts as a link.
(Other definitions for issue that I've seen before include "Matter of debate" , "Hot topic; outcome" , "Offspring or edition" , "that may be taken where there's disagreement?" , "Number (of periodical)" .)