Trick that is used to keep flat in retirement (6)
I believe the answer is:
illude
'trick' is the definition.
(I've seen this before)
'used to keep flat in retirement' is the wordplay.
'used to keep' becomes 'ei' (I am not sure about this - if you are sure you should believe this answer much more).
'flat' becomes 'dull' (synonyms).
'in' indicates putting letters inside.
'retirement' says the letters should be written backwards (retire can mean to return or go back).
'ei' going around 'dull' is 'edulli'.
'edulli' in reverse letter order is 'ILLUDE'.
'that is' is the link.
Can you help me to learn more?
(Another definition for illude that I've seen is " Trick".)