Having eaten the last of entrée, try special nuts (6)
I believe the answer is:
testes
'nuts' is the definition.
('testis' can be a synonym of 'nut')
'having eaten the last of entree try special' is the wordplay.
'having eaten' indicates putting letters inside.
'the last of entree' becomes 'e' (I can't justify this - if you can you should give a lot more credence to this answer).
'try' becomes 'test' (test is a kind of try).
'special' becomes 's' (common abbreviation as in Special Air Service).
'test'+'s'='tests'
'e' inserted within 'tests' is 'TESTES'.
Can you help me to learn more?
(Another definition for testes that I've seen is " balls".)