Brief moment, briefly (4)
'brief moment briefly' is the definition.
(I've seen this in another clue)
This is the entire clue.
'brief moment briefly' is the definition.
(short for second; 'briefly' suggests an abbreviation; 'brief' asks for an abbreviation)
This is all the clue.
(Other definitions for sec that I've seen before include "Jiffy" , "Twinkling" , "sort of function" , "21 down, as wine" , "Dry, like wine" .)