Son fixed gates -- that's most wise (6)
I believe the answer is:
sagest
'most wise' is the definition.
('sage' can be similar in meaning to 'wise')
'son fixed gates' is the wordplay.
'son' becomes 's' (**).
'fixed' is an anagram indicator.
'gates' with letters rearranged gives 'agest'.
's'+'agest'='SAGEST'
'that's' is the link.
(Other definitions for sagest that I've seen before include "Most wise and prudent" , "Extremely clever" , "Wisest" , "Gas set (anag.)" , "Most discerning" .)