Tries at first, when one is a witness (7)
I believe the answer is:
attests
'one is a witness' is the definition.
I can't judge whether this definition defines the answer.
'tries at first' is the wordplay.
'tries' becomes 'tests' (test is a kind of try).
'first' says to put letters next to each other.
'tests' put after 'at' is 'ATTESTS'.
'when' acts as a link.
Can you help me to learn more?
(Other definitions for attests that I've seen before include "Certifies" , "Sat test (anag.)" , "Confirms" , "Bears witness" , "Gives evidence in court" .)