It's Bob's turn to be obstinate (8)
I believe the answer is:
stubborn
'obstinate' is the definition.
(obstinate means firm or stubborn)
'it's bob's turn to be' is the wordplay.
'to be' indicates an anagram.
'bobs'+'turn'='bobsturn'
'bobsturn' anagrammed gives 'STUBBORN'.
(Other definitions for stubborn that I've seen before include "Inflexible" , "Wilful" , "Obstinate, tenacious" , "Obstinate, pig-headed" , "hard to bend" .)