Fastener on a hot-water pipe (6)
I believe the answer is:
hookah
'water pipe' is the definition.
(I know this)
'fastener on a hot' is the wordplay.
'fastener' becomes 'hook' (hook is a kind of fastener).
'on' is a charade indicator (letters next to each other).
'hot' becomes 'h' (abbreviation eg on taps).
'hook'+'a'+'h'='HOOKAH'
(Other definitions for hookah that I've seen before include "Sort of pipe" , "Pipe for smoking marijuana etc" , "Turkish pipe" , "Narcotics pipe" , "Water-pipe" .)