Determined to cook sole? True (8)
I believe the answer is:
resolute
'determined to' is the definition.
(similar in meaning)
'cook sole? true' is the wordplay.
'cook' indicates anagramming the letters.
'sole'+'true'='soletrue'
'soletrue' anagrammed gives 'RESOLUTE'.
(Other definitions for resolute that I've seen before include "'Determined, dead set on (8)'" , "Determined, intent" , "Reels out (anag.)" , "Purposeful and unwavering" , "Determined, firm" .)