Gag judge standing in for start of trial (4)
I believe the answer is:
jest
'gag' is the definition.
(synonyms)
'judge standing in for start of trial' is the wordplay.
'judge' becomes 'j' (abbreviation for judge).
'standing in for' becomes 'es' (I can't justify this - if you can you should believe this answer much more).
'start of' indicates taking the first letters.
The initial letter of 'trial' is 't'.
'j'+'es'+'t'='JEST'
Can you help me to learn more?
(Other definitions for jest that I've seen before include "Humorous remark" , "it's not serious, so to speak" , "Witty remark" , "Jape, witticism" , "Sally" .)