Swear nothing concealed by Head of Audi (German car firm) (4)
I believe the answer is:
avow
'swear' is the definition.
(synonyms)
'nothing concealed by head of audi german car firm' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'concealed by' means one lot of letters goes inside another.
'head of' suggests taking the first letters.
'german car firm' becomes 'vw' (I can't justify this - if you can you should give a lot more credence to this answer).
The initial letter of 'audi' is 'a'.
'a'+'vw'='avw'
'o' going within 'avw' is 'AVOW'.
Can you help me to learn more?
(Other definitions for avow that I've seen before include "Affirm, swear" , "Admit openly" , "Assert - admit" , "Asseverate" , "Declare - affirm" .)