President locks leader of movement in jail (8)
I believe the answer is:
chairman
'president' is the definition.
('chairman' can be a synonym of 'president')
'locks leader of movement in jail' is the wordplay.
'locks' becomes 'hair' (I've seen this before).
'leader of' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'jail' becomes 'can' (the can is American slang for prison).
The initial letter of 'movement' is 'm'.
'hair'+'m'='hairm'
'hairm' going inside 'can' is 'CHAIRMAN'.
(Other definitions for chairman that I've seen before include "official in authority" , "Top person on a committee - this one's male" , "Mao?" , "Committee leader" , "Presiding official" .)