Richard so named 151 females (5)
I believe the answer is:
cliff
'richard so named' is the definition.
(singer Sir Cliff Richard)
'151 females' is the wordplay.
'151' becomes 'cli' (Roman numerals).
'females' becomes 'ff' (I've seen this in other clues).
'cli'+'ff'='CLIFF'
(Other definitions for cliff that I've seen before include "Precipice" , "A sheer rock face, in mountain or by sea" , "Man" , "Sir - - Richard" , "A sheer rock-face" .)