Sluggish emperor is snubbed in Congress (5)
I believe the answer is:
inert
'sluggish' is the definition.
(I know that sluggish can be written as inert)
'emperor is snubbed in congress' is the wordplay.
'emperor' becomes 'nero' (Roman emperor).
'is snubbed' means to remove the last letter (could historically mean cutting the end off).
'in' is an insertion indicator.
'congress' becomes 'it' (congress can mean sex).
'nero' with its last letter taken off is 'ner'.
'ner' put inside 'it' is 'INERT'.
(Other definitions for inert that I've seen before include "Unmoving" , "like neon" , "Like e.g. xenon" , "Lifeless or static" , "like nitrogen" .)