In the nick 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')
'in the nick i lost' is the wordplay.
'in' says the answer is hidden in the clue.
'KILO' is hidden in the letters of 'nick i lost'.
(Other definitions for kilo that I've seen before include "Metric weight, informally" , "Metric prefix meaning 1,000" , "Measure of weight" , "Unit of weight (abbr)" , "Metric unit of weight" .)