Round lake that is least active (6)
I believe the answer is:
idlest
'least active' is the definition.
(I've seen this in another clue)
'round lake that is' is the wordplay.
'round' indicates putting letters inside.
'lake' becomes 'L' (geographical abbreviation).
'that is' becomes 'id est' ('that is' in Latin).
'l' inserted into 'idest' is 'IDLEST'.
(Other definitions for idlest that I've seen before include "Least hard-working" , "Least busy and laziest" , "Least diligent" , "Most useless" , "least fond of Labour?" .)