Mild-mannered chap drained lake (6)
I believe the answer is:
gentle
'mild-mannered' is the definition.
(I've seen this before)
'chap drained lake' is the wordplay.
'chap' becomes 'gent' (I've seen this in other clues).
'drained' suggests removing the centre.
'lake' with its middle taken out is 'le'.
'gent'+'le'='GENTLE'
(Other definitions for gentle that I've seen before include "Kind, not rough" , "Moderate, quiet; a maggot" , "Kind, not harsh" , "Not stern, coarse or violent" , "Mild, not rough" .)