Organ I have held by both hands (5)
I believe the answer is:
liver
'organ' is the definition.
(I know that liver is a type of internal organ)
'i have held by both hands' is the wordplay.
'i have' becomes 'I've' (contraction of 'I have').
'held by' indicates putting letters inside.
'both hands' becomes 'lr' (abbreviations for left and right).
'ive' put within 'lr' is 'LIVER'.
(Other definitions for liver that I've seen before include "Organ; reddish-brown" , "Large organ in the abdomen secreting bile" , "Human organ" , "Detoxifying organ" , "Abdominal organ" .)