Presumably not favouring short stretch (7)

I believe the answer is:

prolong

I'm a little stuck... Click here to teach me more about this clue!

'presumably not' is the definition.
I can't judge whether this definition defines the answer.

'favouring short stretch' is the wordplay.
'favouring' becomes 'pro' (I've seen this in other clues).
'short stretch' becomes 'long' (similar in meaning. I am not sure about the 'short' bit.).
'pro'+'long'='PROLONG'

Can you help me to learn more?

(Other definitions for prolong that I've seen before include "Draw out" , "String out" , "Stretch out" , "Spin out" , "Extend, protract" .)

I've seen this clue in The Telegraph.

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