Children's game is quiet before end of party (1-3)
I believe the answer is:
Here is my best explanation:
'children's game' is the definition.
(I've seen this before)
'is quiet before end of party' is the subsidiary indication.
'quiet' becomes 'p' (A musical term).
'before' says to put letters next to each other.
'end of' indicates one should take the final letters.
The last letter of 'party' is 'y'.
(Other definitions for i-spy that I've seen before include "Game played with young children", "Child's guessing game", "Guessing game", "With my little eye!", "Child's game: ''. ... with my little eye'", "Children's alphabet game", ". ... with my little eye (children's game)", "Wordgame", "Children's game: . ... with my little eye" and "Children's observational game".)