An ulterior motive for what hide-and-go-seek champion might have up his sleeve (6,6)
I believe the answer is:
hidden agenda
'an ulterior motive' is the definition.
(I've seen this before)
'what hide-and-go-seek champion might have up his sleeve' is the wordplay.
I cannot quite see how this works, but
an anagram of 'and' is 'nda' which is present in the answer.
This explanation may well be incorrect...
'for' acts as a link.
Can you help me to learn more?
(Other definitions for hidden agenda that I've seen before include "Concealed intentions" , "Ulterior motive" , "followed in secret meeting?" , "Secret motive behind one's actions" , "Undisclosed reason" .)