Self-absorbed — extra time in first half would make very little difference (10)

I believe the answer is:

egoistical

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'first half would make very little difference' is the definition.
I can't judge whether this defines the answer.

'self-absorbed extra time' is the wordplay.
'self-absorbed' becomes 'egoistic' (similar in meaning).
'extra time' becomes 'al' (I can't explain this - if you can you should give a lot more credence to this answer).
'egoistic'+'al'='EGOISTICAL'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for egoistical that I've seen before include "Self-centred" , "Goalie's tic? (anag)" .)

I've seen this clue in The Guardian.

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