Tap on bottom that may disturb one's rest (6)
I believe the answer is:
bedbug
'that may disturb one's rest' is the definition.
(bedbugs might disturb someone's sleep)
'tap on bottom' is the wordplay.
'tap' becomes 'bug' (synonyms).
'on' is a charade indicator (letters next to each other).
'bottom' becomes 'bed' (synonyms).
'bug' put after 'bed' is 'BEDBUG'.
(Other definitions for bedbug that I've seen before include "Bloodsucker's" , "It sucks your blood while you sleep" , "Insect in the mattress, perhaps?" , "Night-time bloodsucker" , "Parasite between the sheets?" .)