Utter justification to lose weight (6)
I believe the answer is:
arrant
'utter' is the definition.
(old-fashioned term meaning complete or utter)
'justification to lose weight' is the wordplay.
'justification' becomes 'warrant'.
'to lose' indicates named letters should be taken away.
'weight' becomes 'w' (short for weight).
'warrant' with 'w' taken away is 'ARRANT'.
(Other definitions for arrant that I've seen before include "Out-and-out, maybe nonsense" , "Utter, complete" , "Out-and-out, notorious" , "Utter; downright" , "Out-and-out, utter" .)