Unfortunate slip of the lips (4)
I believe the answer is:
lisp
'unfortunate slip' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'of the lips' is the wordplay.
'of' indicates an anagram (the answer comes from or 'of' the letters).
'lips' anagrammed gives 'LISP'.
Can you help me to learn more?
(Other definitions for lisp that I've seen before include "Non-standard pronunciation" , "speak imperfectly" , "Speak with defective sibilants" , "Be unable to pronounce S" , "Thpeak like thith" .)