Where credibility helps? It may stand without further probe, ultimately (6)
I believe the answer is:
street
'where credibility helps? it' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'stand without further probe ultimately' is the wordplay.
'stand' becomes 'stet' ().
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'ultimately' indicates one should take the final letters.
The final letters of 'further probe' are 're'.
'stet' placed around 're' is 'STREET'.
'may' is the link.
Can you help me to learn more?
(Other definitions for street that I've seen before include "See 1 Down" , "Tester (anag.)" , "Tester in public road" , "Public road in a conurbation" , "Built-up road" .)