Dirty introduction to sex with lubricant (4)
I believe the answer is:
soil
'dirty' is the definition.
('soil' can be a synonym of 'dirty')
'introduction to sex with lubricant' is the wordplay.
'introduction to' indicates taking the first letters (I've seen 'introductions to' mean this).
'with' means one lot of letters go next to another.
'lubricant' becomes 'oil' (I've seen this before).
The first letter of 'sex' is 's'.
's'+'oil'='SOIL'
(Other definitions for soil that I've seen before include "Earth for growing" , "refuse" , "Top layer of the surface of the earth" , "Earth; stain" , "Loose upper layer of earth" .)