Sun protection factor over 50 if one gets burnt (6)

I believe the answer is:

spliff

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'sun protection factor' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'50 if one gets burnt' is the wordplay.
'50' becomes 'l' (Roman numeral).
'one gets' means one lot of letters goes inside another.
'burnt' becomes 'spf' (I can't justify this - if you can you should give a lot more credence to this answer).
'l'+'if'='lif'
'lif' inserted within 'spf' is 'SPLIFF'.

'over' acts as a link.

Can you help me to learn more?

(Other definitions for spliff that I've seen before include "Blow in this tube" , "Cannabis cigarette" , "Dodgy smoke" , "drug dose" , "Marijuana cigarette" .)

I've seen this clue in The Guardian.

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