Sack dunderhead - is spinster (7)

I believe the answer is:

dismiss

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'sack' is the definition.
(I know that sack can be written as dismiss)

'dunderhead is spinster' is the wordplay.
'dunderhead' becomes 'd' (head letter of 'dunder').
'spinster' becomes 'miss' (I am not sure about this - if you are sure you should believe this answer much more).
'd'+'is'+'miss'='DISMISS'

Can you help me to learn more?

(Other definitions for dismiss that I've seen before include "Throw out" , "Sack - send away" , "military order" , "Fire - cashier" , "Sack - cashier" .)

I've seen this clue in The Irish Independent.

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