Mishandle our treatment when underfed (12)
I believe the answer is:
malnourished
'underfed' is the definition.
(similar in meaning)
'mishandle our treatment' is the wordplay.
'treatment' is an anagram indicator (letters treated or processed differently).
'mishandle'+'our'='mishandleour'
'mishandleour' is an anagram of 'MALNOURISHED'.
'when' is the link.
(Other definitions for malnourished that I've seen before include "Underfed" , "Provided with inadequate sustenance" , "poorly fed" , "Starved" .)