Hack making computer's first operating system stall (5)
I believe the answer is:
kiosk
'stall' is the definition.
(synonyms)
'hack making computer's first operating system' is the wordplay.
I cannot really understand how this works, but
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is found in the answer.
'operating system' could be 'os' (synonyms) and 'os' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for kiosk that I've seen before include "Small sales booth" , "Newsstand - refreshment outlet" , "Roofed stall; telephone box" , "here's where to get tickets" , "Pavement shop" .)