Second traveller swallows tablet on top of that (8)
I believe the answer is:
moreover
'on top of that' is the definition.
(both can be used to introduce an additional point)
'second traveller swallows tablet' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'traveller' becomes 'rover' (rover is a kind of traveller).
'swallows' is an insertion indicator.
'tablet' becomes 'e' (E can mean ecstasy or an ecstasy tablet).
'rover' placed around 'e' is 'reover'.
'mo'+'reover'='MOREOVER'
(Other definitions for moreover that I've seen before include "Furthermore, in addition" , "as well" , "Besides, in addition" .)