Regularly missing usual uncle’s piquancy (5)
I believe the answer is:
sauce
'piquancy' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'regularly missing usual uncle's' is the wordplay.
'regularly missing' indicates alternate letters (letters removed at regular intervals).
'usual'+'uncles'='usualuncles'
The alternating letters of 'usualuncles' are 'SAUCE'.
Can you help me to learn more?
(Other definitions for sauce that I've seen before include "Relish or impertinence" , "Cause (anag.)" , "Worcester, say" , "Impertinence; dressing" , "Insolence" .)