Blue livery with top half removed -- that's cheating (8)
I believe the answer is:
adultery
'cheating' is the definition.
(cheating on a partner)
'blue livery with top half removed' is the wordplay.
'blue' becomes 'adult' (blue can mean pornographic or adult).
'with top half removed' means to take half the letters.
'livery' cut in half is 'ery'.
'adult'+'ery'='ADULTERY'
'that's' acts as a link.
(Other definitions for adultery that I've seen before include "frowned-on relations" , "Extramarital sex" , "Act of infidelity by husband or wife" , "Betrayal of a partner" , "cheating" .)