Round lake that is most inactive (6)
I believe the answer is:
idlest
'most inactive' is the definition.
('idle' can be similar in meaning to 'inactive')
'round lake that is' is the wordplay.
'round' is an insertion indicator.
'lake' becomes 'L' (geographical abbreviation).
'that is' becomes 'id est' ('that is' in Latin).
'l' placed inside 'idest' is 'IDLEST'.
(Other definitions for idlest that I've seen before include "Most lazy" , "Least diligent" , "Least hard-working" , "most unused" , "making minimal effort" .)