Determined supporter boring Sylvester Stallone (6)
I believe the answer is:
steely
'determined' is the definition.
(I've seen this before)
'supporter boring sylvester stallone' is the wordplay.
I cannot quite see how this works, but
'supporter' could be 'tee' (a golf tee acts as a support) and 'tee' is found within the answer.
'stallone' could be 'sly' (nickname for Sylvester Stallone) and 'sly' is present in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for steely that I've seen before include "Strong" , "Set off" , "With (cold) resolve" , "Gimlet-eyed" , "Inflexibly severe" .)