Happy to get new organ (5)
I believe the answer is:
gland
'organ' is the definition.
(gland is a kind of organ)
'happy to get new' is the wordplay.
'happy' becomes 'glad' (similar in meaning).
'to get' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'glad' enclosing 'n' is 'GLAND'.
(Other definitions for gland that I've seen before include "Organ that separates substances from blood" , "it could be described as secretive" , "liver, for example" , "Organ that releases essential substances into the bloodstream" , "Organ secreting chemical substances" .)