Dad sets out to be bluest (7)
I believe the answer is:
saddest
'be bluest' is the definition.
('sad' can be similar in meaning to 'blue')
'dad sets out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'dad'+'sets'='dadsets'
'dadsets' with letters rearranged gives 'SADDEST'.
'to' acts as a link.
(Other definitions for saddest that I've seen before include "Most unhappy" , "most dejected" , "Unhappiest" , "Most doleful" , "Most gloomy" .)