Standard indication of airworthiness (4,4)
I believe the answer is:
kite mark
'airworthiness' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see an association between them that I can't see?
'standard indication' is the wordplay.
'standard' becomes 'kite' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'indication' becomes 'mark' (mark is a kind of indication).
'kite'+'mark'='KITE MARK'
'of' is the link.
Can you help me to learn more?
(Other definitions for kite mark that I've seen before include "Sign that British goods meet official standards" , "Sign of quality assurance" , "British Standards Institution's symbol of approval" , "Indication of approval from BSI" , "British quality trade mark" .)