Stubborn old boy expected to cross grass (8)
I believe the answer is:
obdurate
'stubborn' is the definition.
(I've seen this before)
'old boy expected to cross grass' is the wordplay.
'old boy' becomes 'ob' (abbreviation).
'expected' becomes 'due' (similar in meaning).
'to cross' is an insertion indicator.
'grass' becomes 'rat' (both can mean a snitch or informer).
'due' placed around 'rat' is 'durate'.
'ob'+'durate'='OBDURATE'
(Other definitions for obdurate that I've seen before include "Stubbornly persistent" , "Hard-hearted" , "Intractable" , "Impervious to persuasion" , "Not to be persuaded" .)