A magic word to keep uncle in check (6)
I believe the answer is:
sesame
'a magic word to keep' is the definition.
I can't tell whether this definition defines the answer.
'uncle in check' is the wordplay.
'uncle' becomes 'sam' (Uncle Sam is an example).
'in' means one lot of letters goes inside another.
'check' becomes 'see' (synonyms).
'sam' going inside 'see' is 'SESAME'.
Can you help me to learn more?
(Other definitions for sesame that I've seen before include "Seeds used for making oil" , "Plant yielding seeds used for flavouring bread" , "Plant cultivated for its seeds - opening word?" , "Plant with edible seeds" , "Open ...... is magic phrase for opening door in pantomime" .)