Postholder lacks aim somehow (8)

I believe the answer is:

mailsack

I'm a little stuck... Click here to teach me more about this clue!

'postholder' is the definition.
I can't tell whether this defines the answer.

'lacks aim somehow' is the wordplay.
'somehow' is an anagram indicator.
'lacks'+'aim'='lacksaim'
'lacksaim' anagrammed gives 'MAILSACK'.

Can you help me to learn more?

(Another definition for mailsack that I've seen is " Pat carted it round?".)

I've seen this clue in The Times.

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