Ruin second milliner (7)
I believe the answer is:
shatter
'ruin' is the definition.
(I've seen this before)
'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 "Completely upset" , "Break into many pieces" , "thoroughly upset" , "Break into many bits" , "Crack" .)