Idle MD's developing waist (6)
I believe the answer is:
middle
'waist' is the definition.
('middle' can be a synonym of 'waist')
'idle md's developing' is the wordplay.
'developing' indicates an anagram.
'idle'+'md'='idlemd'
'idlemd' is an anagram of 'MIDDLE'.
(Other definitions for middle that I've seen before include "Centre or core" , "Compromise position" , "Median" , "Intermediate" , "(A person's) waist" .)