A longing that Cockney fails to express? (5)
I believe the answer is:
aitch
'a longing that cockney fails to express?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'a longing' is the wordplay.
'longing' becomes 'itch' (itch can mean a desire or longing).
'a'+'itch'='AITCH'
Can you help me to learn more?
(Other definitions for aitch that I've seen before include "The letter h in full" , "first bit of hymn?" , "Aspirated letter" , "bone" , "Letter sound that's often dropped?" .)