To eat, a portion of trifle would be really nice (5)
I believe the answer is:
treat
'trifle would be really nice' is the definition.
I can't tell whether this definition defines the answer.
'to eat a portion' is the wordplay.
'to' becomes 'tat' (I can't justify this - if you can you should believe this answer much more).
'eat' means one lot of letters goes inside another.
'a portion' becomes 're' (re is a kind of portion).
'tat' placed around 're' is 'TREAT'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for treat that I've seen before include "Entertain - minister - free entertainment" , "Minister - buy one a drink" , "Pay for someone's food or drink or look after them medically" , "Behave towards" , "Shout" .)