At 51, hard-hearted deceiver (4)
I believe the answer is:
liar
'deceiver' is the definition.
(I know that liar is a type of deceiver)
'at 51 hard-hearted' is the wordplay.
'at' is a charade indicator (letters next to each other).
'51' becomes 'LI' (Roman numerals).
'hearted' means to look at the middle letters.
The middle of 'hard' is 'ar'.
'li'+'ar' is 'LIAR'.
(Other definitions for liar that I've seen before include "Rail at a prevaricator" , "Perjurer" , "Dissembler" , "Person telling stories" , "not straight [one]" .)