Swears at trials (7)
I believe the answer is:
attests
'swears' is the definition.
(I know this)
'at trials' is the wordplay.
'trials' becomes 'tests' ('test' can be a synonym of 'trial').
'at'+'tests'='ATTESTS'
(Other definitions for attests that I've seen before include "Authenticates or affirms to be true" , "Sat test (anag.)" , "Bears witness" , "Certifies" , "Gives evidence in court" .)