Nasty bloke, putting head back, crossing line (4)
I believe the answer is:
ugly
'nasty' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
(Other definitions for ugly that I've seen before include "describe duckling thus" , "Not nice-looking" , "Shade of hat" , "Sullen" , "It's not fair" .)