Most risqué image infiltrates short nap (8)
I believe the answer is:
spiciest
'most risque' is the definition.
('spicy' can be a synonym of 'risque')
'image infiltrates short nap' is the wordplay.
'image' becomes 'pic' (short for picture).
'infiltrates' is an insertion indicator.
'short' means to remove the last letter.
'nap' becomes 'siesta' (siesta is a kind of nap).
'siesta' with its last letter removed is 'siest'.
'pic' placed inside 'siest' is 'SPICIEST'.
(Another definition for spiciest that I've seen is " Most seasoned - try it with Pisces".)