It's a dirty lie! (5)
I believe the answer is:
clean
'dirty lie' is the definition.
The definition and answer can be both to do with contact as well as being verbs in their base form.
Maybe you can see a link between them that I don't see?
'it's a' is the wordplay.
'it's' becomes 'cle' (I can't justify this - if you can you should believe this answer much more).
'a' becomes 'an'.
'cle'+'an'='CLEAN'
Can you help me to learn more?
(Other definitions for clean that I've seen before include "Second part of 18" , "Absolute - pure" , "Spotless; launder" , "Not obscene; smooth" , "Free from dirt; well defined" .)