A twice-married woman's outside clobber (6)
I believe the answer is:
hammer
'clobber' is the definition.
(I've seen this before)
'a twice-married woman's outside' is the wordplay.
'twice' shows that letters should be duplicated.
'married' becomes 'm' (abbreviation, e.g. in genealogy).
'woman' becomes 'her' (her refers to a woman).
'outside' means one lot of letters goes inside another.
'm' doubled is 'mm'.
'a'+'mm'='amm'
'amm' inserted into 'her' is 'HAMMER'.
(Other definitions for hammer that I've seen before include "A striking tool for blacksmith" , "See 24" , "It's applied to a nail" , "Auctioneer's gavel" , "Tool; athletic event" .)