Devastate second milliner (7)
I believe the answer is:
shatter
'devastate' is the definition.
(thesaurus)
'second milliner' is the wordplay.
'second' becomes 's' (common abbreviation).
'milliner' becomes 'hatter' (synonyms).
's'+'hatter'='SHATTER'
(Other definitions for shatter that I've seen before include "Support for broken arm" , "greatly upset" , "thoroughly upset" , "Break into fragments" , "Ruin" .)