Happy with new organ (5)
I believe the answer is:
gland
'organ' is the definition.
(gland is a kind of organ)
'happy with new' is the wordplay.
'happy' becomes 'glad' (synonyms).
'with' is an insertion indicator.
'new' becomes 'n' (abbreviation as in New Testament NT).
'glad' placed around 'n' is 'GLAND'.
(Other definitions for gland that I've seen before include "Organ that secretes" , "it could be described as secretive" , "vital structure" , "Organ that releases essential substances into the bloodstream" , "Organ secreting chemical substances" .)