Touching lightly so as to cause laughter (8)
I believe the answer is:
tickling
'touching lightly so as to cause laughter' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tickling that I've seen before include "Action by angler" , "Catching (trout) by rubbing with the hand" , "Touching lightly to cause laughter" .)