Joke in Seinfeld occasionally being used by someone else (7)
I believe the answer is:
engaged
'used by someone else' is the definition.
I can't tell whether this definition defines the answer.
'joke in seinfeld occasionally' is the wordplay.
'joke' becomes 'gag' (a gag is a joke or witticism).
'in' is an insertion indicator.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
The alternating letters of 'seinfeld' are 'ened'.
'gag' going into 'ened' is 'ENGAGED'.
'being' is the link.
Can you help me to learn more?
(Other definitions for engaged that I've seen before include "Not free" , "Held" , "Spoken for" , "Preoccupied" , "Took on" .)