Holy man and Yankee cross legendary river (4)
I believe the answer is:
styx
'legendary river' is the definition.
(I know this)
'holy man and yankee cross' is the wordplay.
'holy man' becomes 'st' (abbreviation for saint).
'and' is a charade indicator (letters next to each other).
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'cross' becomes 'x' (an X is a cross).
'st'+'y'+'x'='STYX'
(Other definitions for styx that I've seen before include "classical stream" , "Hellish current" , "Hell's river" , "The last river one crosses" , "River crossed at death" .)