Amaze a firm over time (7)

I believe the answer is:

astound

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'amaze' is the definition.
(synonyms)

'a firm over time' is the wordplay.
'firm' becomes 'sound' (I can't justify this - if you can you should believe this answer much more).
'over' is an insertion indicator.
'time' becomes 't' (abbreviation).
'sound' placed around 't' is 'stound'.
'a'+'stound'='ASTOUND'

Can you help me to learn more?

(Other definitions for astound that I've seen before include "Shock, surprise greatly" , "Stagger" , "Fill with wonder or astonishment" , "Amaze, astonish" , "Flabbergast" .)

I've seen this clue in The Telegraph.

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