"One ___ . . ." (3)
'one' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for unit that I've seen before include "A considerable number of soldiers" , "One; military group" , "Single manufactured item; military grouping" , "Individual thing or group regarded as complete" , "Module" .)
(Other definitions for sec that I've seen before include "Dry wine perhaps" , "Moderately dry, champagne say" , "sparkling?" , "(Of wines) dry" , "period not lasting long" .)