Bob turns nasty and pig-headed (8)
I believe the answer is:
stubborn
'pig-headed' is the definition.
(I've seen this before)
'bob turns nasty' is the wordplay.
'nasty' indicates anagramming the letters.
'bob'+'turns'='bobturns'
'bobturns' is an anagram of 'STUBBORN'.
'and' is the link.
(Other definitions for stubborn that I've seen before include "Obstinate, bullheaded" , "Showing dogged determination" , "Obdurate and unyielding (8)" , "Tenaciously unwilling to yield" , "Difficult to move" .)