Fixing undiluted drink, suddenly left, upset (8)
I believe the answer is:
rawlplug
'fixing' is the definition.
(trademark name for a type of fixing)
'undiluted drink suddenly left upset' is the wordplay.
'undiluted' becomes 'raw'.
'drink suddenly' becomes 'gulp'.
'left' becomes 'l' (common abbreviation).
'upset' shows that the letters should be reversed in order.
'gulp'+'l'='gulpl'
'gulpl' backwards is 'lplug'.
'raw'+'lplug'='RAWLPLUG'
(Other definitions for rawlplug that I've seen before include "Plastic sheath to hold a screw in a masonry hole" , "Fixer for screw in wall" , "Means of fixing screw to wall" , "Means of putting screw in wall" , "Small tube" .)