Missing the first biggest show (6)
I believe the answer is:
attest
'show' is the definition.
(attesting is a kind of showing)
'missing the first biggest' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'first' could be 'a' and 'a' is located in the answer.
The remaining letters 'test' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for attest that I've seen before include "Declare to be true or genuine" , "Confirm to be true" , "Give testimony in a court of law" , "Bear witness, declare to be true" , "Certify or bear witness" .)