The shortcoming of a left back who's lost his head (4)

I believe the answer is:

lack

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'the shortcoming' is the definition.
(thesaurus)

I can't explain the rest of the clue.

(Other definitions for lack that I've seen before include "Be without, need" , "Absence, deficiency" , "Be wanting or without" , "Be without or short of" , "Deficiency (of something)" .)

I've seen this clue in the Evening Standard.

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