Solitary ace held in both hands (5)
I believe the answer is:
loner
'solitary' is the definition.
(I know that solitary can be written as loner)
'ace held in both hands' is the wordplay.
'ace' becomes 'one' (synonyms).
'held in' is an insertion indicator.
'both hands' becomes 'LR' (abbreviations for left and right).
'one' inserted inside 'lr' is 'LONER'.
(Other definitions for loner that I've seen before include "Solitary person" , "One shunning company" , "Solitary type" , "Individualist" , "Unsociable person" .)