Act to capture constant highwayman (6)
I believe the answer is:
turpin
'highwayman' is the definition.
(I know that Turpin is a highwayman)
'act to capture constant' is the wordplay.
'act' becomes 'turn' (theatre).
'to capture' means one lot of letters goes inside another.
'constant' becomes 'pi' (mathematical constant).
'turn' placed around 'pi' is 'TURPIN'.
(Other definitions for turpin that I've seen before include "Dick.....was famous English highwayman" , "Dick ...... was a legendary highwayman" , "One who preyed on travellers" , "Dick ... was notorious English highwayman" , "Dick ... was famous English highwayman" .)