Small milliner greatly upset (7)
I believe the answer is:
shatter
'greatly upset' is the definition.
(I've seen this before)
'small milliner' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'milliner' becomes 'hatter' (synonyms).
's'+'hatter'='SHATTER'
(Other definitions for shatter that I've seen before include "greatly upset" , "Break into many bits" , "wreck" , "Completely upset" , "Break into many pieces" .)