Touch on (4)
I believe the answer is:
abut
'touch on' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for abut that I've seen before include "Lean against the odd tuba" , "Be next door to" , "Adjoin, be contiguous to" , "Adjoin, touch" , "Adjoin, press up against" .)