Eccentric directors, lacking depth (9)

I believe the answer is:

cardboard

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'lacking depth' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'eccentric directors' is the wordplay.
'eccentric' becomes 'card' (I've seen this before).
'directors' becomes 'board' (board of directors).
'card'+'board'='CARDBOARD'

Can you help me to learn more?

(Other definitions for cardboard that I've seen before include "without substance" , "A stiff thick paper" , "stiff material" .)

I've seen this clue in The Daily Express.

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