Mild-mannered man drained lake (6)
I believe the answer is:
gentle
'mild-mannered' is the definition.
(I've seen this before)
'man drained lake' is the wordplay.
'man' becomes 'gent' (gent is a kind of man).
'drained' says to hollow out the word (remove centre letters).
'lake' with its centre taken out is 'le'.
'gent'+'le'='GENTLE'
(Other definitions for gentle that I've seen before include "Mild and considerate" , "'Do not go . . . . . . into that good night' (Dylan Thomas)" , "Not stern, coarse or violent" , "Mild, not rough" , "Kind - maggot" .)