Slightly alter poster before fair (6)

I believe the answer is:

adjust

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'slightly alter' is the definition.
'adjust' can be an answer for 'alter' (I've seen this before). I am not sure about the 'slightly' bit.

'poster before fair' is the wordplay.
'poster' becomes 'ad'.
'before' means one lot of letters go next to another.
'fair' becomes 'just' (associated in meaning).
'ad'+'just'='ADJUST'

Can you help me to learn more?

(Other definitions for adjust that I've seen before include "Adapt, fix" , "Calibrate or regulate" , "Change slightly to fit" , "Adapt, regulate" , "Arrange, put into proper position" .)

I've seen this clue in The Independent.

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