Tipped fast in sound (5)

I believe the answer is:

leant

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'tipped fast in sound' is the definition.
'leant' can be an answer for 'tipped' (I've seen this before). I am unsure of the 'fast in sound' bit.

'tipped fast in' is the wordplay.
'tipped' becomes 'a' (I can't explain this - if you can you should give a lot more credence to this answer).
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'in' is an insertion indicator.
'a' placed inside 'lent' is 'LEANT'.

Can you help me to learn more?

(Other definitions for leant that I've seen before include "Slanted" , "Rested sideways (on something)" , "Inclined (towards)" , "bent over" , "Tipped" .)

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

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