Festivity a habitual convict upset (4)

I believe the answer is:

gala

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'festivity' is the definition.
(I know that festivity can be written as gala)

'a habitual convict upset' is the wordplay.
'habitual convict' becomes 'lag' (I've seen this before. I am not sure about the 'habitual' bit.).
'upset' shows that the letters should be reversed in order.
'a'+'lag'='alag'
'alag' written backwards gives 'GALA'.

Can you help me to learn more?

(Other definitions for gala that I've seen before include "Sports meeting" , "Mix" , "Festive celebration" , "Festivity - celebration" , "Festive entertainment" .)

I've seen this clue in The Times.

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