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' means one lot of letters goes inside another.
'congress' becomes 'it' (congress can mean sex).
'nero' with its final letter taken away is 'ner'.
'ner' placed into '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" .)