Understand what’s primarily involved in kid’s game (4)
I believe the answer is:
twig
'understand' is the definition.
(I've seen this before)
'what's primarily involved in kid's game' is the wordplay.
'primarily' says to take the initial letters.
'involved in' means one lot of letters goes inside another.
'kid's game' becomes 'tig' (I can't explain this - if you can you should believe this answer much more).
The first letter of 'whats' is 'w'.
'w' placed inside 'tig' is 'TWIG'.
Can you help me to learn more?
(Other definitions for twig that I've seen before include "growth of smaller branches" , "Suddenly get" , "Slender woody shoot" , "Small shoot or branch" , "Informally grasp meaning" .)