Tight dress? Keep quiet, suppressing passion (6)
I believe the answer is:
sheath
'tight dress?' is the definition.
(I've seen this in another clue)
'keep quiet suppressing passion' is the wordplay.
'keep quiet' becomes 'sh' (as in shushing someone).
'suppressing' is an insertion indicator.
'passion' becomes 'heat' (synonyms).
'sh' placed around 'heat' is 'SHEATH'.
(Other definitions for sheath that I've seen before include "cocktail dress" , "Case for a dagger" , "Casing for a sword" , "Cover protecting" , "coat" .)