I mess about when lapsed Puritan (10)

I believe the answer is:

abstemious

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'lapsed puritan' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

'i mess about' is the wordplay.
'i mess about' can be anagrammed to 'ABSTEMIOUS'.
However, I'm unsure how the anagram is indicated.

'when' acts as a link.

Can you help me to learn more?

(Other definitions for abstemious that I've seen before include "Temperate" , "refraining from excess" , "Indulging only moderately (in food/drink)" , "Self-denying" , "not eating or drinking a lot" .)

I've seen this clue in The Telegraph.

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