Children’s game this pyramid conceals (1-3)
I believe the answer is:
i-spy
'childrens game' is the definition.
(I've seen this before)
'this pyramid conceals' is the subsidiary indication.
'conceals' indicates a hidden word.
'ispy' can be found hidden inside 'this pyramid'.
(Other definitions for i-spy that I've seen before include "Child's game: ''. ... with my little eye'" , "Children's alphabet game" , "Child's guessing game" , "Children's observational game" , ". ... with my little eye (children's game)" .)