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' (looks like zero - 0).
'concealed by' indicates putting letters inside.
'head of' says to take the initial letters.
'german car firm' becomes 'vw' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The initial letter of 'audi' is 'a'.
'a'+'vw'='avw'
'o' put into '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" .)