Previously in sight of assistance (10)

I believe the answer is:

beforehand

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

'in sight of assistance' is the wordplay.
'in sight of' becomes 'before' (I've seen this before).
'assistance' becomes 'hand' (hand is a kind of assistance).
'before'+'hand'='BEFOREHAND'

(Other definitions for beforehand that I've seen before include "Previously" , "Earlier" , "In advance" .)

I've seen this clue in The Telegraph.

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