A little over 51? That's special powers (9)
I believe the answer is:
abilities
'powers' is the definition.
('ability' can be a synonym of 'power')
'a little over 51? that's special' is the wordplay.
'a little' becomes 'abit' (synonyms).
'over' is an insertion indicator.
'5a?' becomes 'LI' (Roman numerals).
'that's' becomes 'ie'.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'abit' enclosing 'li' is 'abilit'.
'abilit'+'ie'+'s'='ABILITIES'
(Other definitions for abilities that I've seen before include "skills" , "Talents" , "The powers to do" .)