Fashionable, irrational surrealist painters acting as if related (2,4,8)
I believe the answer is:
in loco parentis
'acting as if related' is the definition.
I can't tell whether this definition defines the answer.
'fashionable irrational surrealist painters' is the wordplay.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'irrational' becomes 'loco' (I am not sure about this - if you are sure you should believe this answer much more).
'surrealist' indicates an anagram (I've seen this in other clues).
'painters' anagrammed gives 'parentis'.
'in'+'loco'+'parentis'='IN LOCO PARENTIS'
Can you help me to learn more?
(Other definitions for in loco parentis that I've seen before include "As a teacher may be" , "In the place of a parent" , "Trying to be mum" , "instead of the parents" , "with kids under his wing?" .)