Horribly snide reaction being tactless (13)
I believe the answer is:
inconsiderate
'tactless' is the definition.
(associated in meaning)
'horribly snide reaction' is the wordplay.
'horribly' indicates an anagram (a horribly wrong version of the letters).
'snide'+'reaction'='snidereaction'
'snidereaction' anagrammed gives 'INCONSIDERATE'.
'being' acts as a link.
(Other definitions for inconsiderate that I've seen before include "Heedless" , "intolerant" , "Lacking regard for the feelings of others" , "Thoughtless, rash" , "Self-centred" .)