When sister gets to beat backside of current helper? (9)

I believe the answer is:

assistant

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'current helper?' is the definition.
'assistant' can be an answer for 'helper?' (synonyms). I am not certain of the 'current' bit.

'when sister gets to beat backside' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'sister' becomes 'sis' (synonyms).
'gets' says to put letters next to each other.
'to beat backside' becomes 'tant' (I can't explain this - if you can you should believe this answer much more).
'as'+'sis'+'tant'='ASSISTANT'

'of' is the link.

Can you help me to learn more?

(Other definitions for assistant that I've seen before include "Deputy, subordinate" , "Aide, helper" , "help required?" , "one helps out" , "second" .)

I've seen this clue in The Guardian.

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