Children's game is extremely pretty (1-3)
I believe the answer is:
i-spy
'children's game' is the definition.
(I Spy is a guessing game)
'is extremely pretty' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'pretty' with its centre taken out is 'py'.
'is'+'py'='I-SPY'
(Other definitions for i-spy that I've seen before include "Children's alphabet game" , ". ... with my little eye (children's game)" , "Game played with young children" , "Children's guessing game" , "Wordgame" .)