Within a week I lost weight (4)
I believe the answer is:
kilo
'weight' is the definition.
(I have seen 'Metric weight' mean 'kilo' so perhaps 'weight' could also mean 'kilo')
'within a week i lost' is the wordplay.
'within' says the answer is hidden in the clue.
'KILO' is hidden within 'week i lost'.
(Other definitions for kilo that I've seen before include "2.2 lbs" , "Measure of weight" , "Unit of weight (abbr)" , "Approx 2 point 205 lbs" , "Metric prefix meaning 1,000" .)