Most of a truism about computing performance (3)
I believe the answer is:
gig
'performance' is the definition.
(I've seen this before)
'most of a truism about computing' is the wordplay.
'most of' means to remove the last letter.
'a truism about computing' becomes 'gigo' (short for data truism 'garbage in, garbage out').
'gigo' with its last letter taken away is 'GIG'.
(Other definitions for gig that I've seen before include "Live performance by musicians" , "One-horse carriage" , "Live musical performance" , "A one-night engagement for entertainer" , "Band performance" .)