The moment king enters, fetch seal (6)
I believe the answer is:
gasket
'seal' is the definition.
(I know that gasket is a type of seal)
'the moment king enters fetch' is the wordplay.
'the moment king' becomes 'ask' (I can't explain this - if you can you should believe this answer much more).
'enters' means one lot of letters goes inside another.
'fetch' becomes 'get' (synonyms).
'ask' put inside 'get' is 'GASKET'.
Can you help me to learn more?
(Other definitions for gasket that I've seen before include "It provides a seal" , "Compressible piece of rubber forming a seal" , "packing material" , "sandwich filling" , "Flat metal seal between tow surfaces as of engine" .)