Most intemperate to opt out of optometrists' scheme (9)
I believe the answer is:
stormiest
'most intemperate' is the definition.
('stormy' can be similar in meaning to 'intemperate')
'opt out of optometrists' scheme' is the wordplay.
'out of' is a deletion indicator.
'scheme' indicates an anagram.
'optometrists' with 'opt' taken away is 'ometrists'.
'ometrists' is an anagram of 'STORMIEST'.
'to' acts as a link.
(Other definitions for stormiest that I've seen before include "Most turbulent" , "Tries most (anag.)" .)