Decreases 40 per cent of lines in text (5)

I believe the answer is:

slims

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'decreases 40 per cent' is the definition.
I can't judge whether this definition defines the answer.

'of lines in text' is the wordplay.
'of lines' becomes 'li' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'text' becomes 'SMS' (abbreviation for Short Message Service).
'li' inserted within 'sms' is 'SLIMS'.

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I've seen this clue in The Times.

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