Dismiss ultimately deficient retainer (3)
I believe the answer is:
sac
'retainer' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'dismiss ultimately deficient' is the wordplay.
'dismiss' becomes 'sack' (synonyms).
'ultimately deficient' means to remove the last letter (the word lacks its last or ultimate letter).
'sack' with its last letter taken away is 'SAC'.
Can you help me to learn more?
(Other definitions for sac that I've seen before include "Traditional entitlement to hold court" , "pocket" , "Pouch or bag-like structure" , "Animal or vegetable pouch" , "Bag, pouch; cyst" .)