Mention in passing (5,2)
'mention in passing' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for allude that I've seen before include "Mention; imply" , "Make reference to in passing" , "Suggest or call attention to indirectly" , "Speak about in passing" , "Refer to, speak of" .)
(Another definition for touch on that I've seen is " Mention briefly in tactile manner?".)