Allergic reaction having taken LSD, perhaps: expression of support welcomed (11)
I believe the answer is:
intolerance
'allergic reaction' is the definition.
(eg gluten intolerance)
'taken lsd perhaps expression of support welcomed' is the wordplay.
I cannot quite see how this works, but
'expression' could be 'into' (I have seen 'Expression of division ' mean 'into' so perhaps 'expression' could also mean 'into') and 'into' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'having' acts as a link.
Can you help me to learn more?
(Other definitions for intolerance that I've seen before include "bigotry" , "illiberal attitude" , "D W Griffith film" , "antipathy to different views" , "Inability to endure (something)" .)