President locks maiden in prison (8)
I believe the answer is:
chairman
'president' is the definition.
('chairman' can be a synonym of 'president')
'locks maiden in prison' is the wordplay.
'locks' becomes 'hair' (I've seen this before).
'maiden' becomes 'm' (cricket abbreviation).
'in' means one lot of letters goes inside another.
'prison' becomes 'can' (the can is American slang for prison).
'hair'+'m'='hairm'
'hairm' inserted into 'can' is 'CHAIRMAN'.
(Other definitions for chairman that I've seen before include "Committee, club, head" , "Boss" , "Person presiding over a board" , "Officer who presides at meetings" , "Casting voter" .)