Past master at fistful of fish? (3,4)

I believe the answer is:

dab hand

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'past master' is the definition.
(I've seen this before)

'at fistful of fish?' is the wordplay.
'at' means one lot of letters go next to another.
'fistful of' becomes 'hand' (I can't justify this - if you can you should believe this answer much more).
'fish?' becomes 'dab' (dab is a type of fish).
'hand' after 'dab' is 'DAB HAND'.

Can you help me to learn more?

(Other definitions for dab hand that I've seen before include "cursory wash?" , "Barely wash" , "Past master" , "An informal expert such as band had" .)

I've seen this clue in the Sydney Morning Herald.

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