Brief moment
'brief moment' is the definition.
(short for second; 'brief' indicates an abbreviation)
This is all the clue.
(Other definitions for sec that I've seen before include "'Dry, as wine (3)'" , "Mo or min?" , "(Of wines) dry" , "21 down, as wine" , "very short space of time" .)
(Other definitions for bit that I've seen before include "Small portion - part of drill" , "a curb?" , "Between horse's teeth, this loses control" , "A little coin" , "Boring tip of a drill" .)
'brief moment' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for instant that I've seen before include "Not detailed" , "Short time, of the current month" , "type of coffee" , "Trice" , "The blink of an eye" .)