Curved sword usually triangular originally in form? (7)
I believe the answer is:
cutlass
'curved sword' is the definition.
(I've seen this before)
'usually triangular originally in form?' is the wordplay.
'originally' indicates taking the first letters.
'in' is an insertion indicator.
'form?' becomes 'class' ('class' can be a synonym of 'form'**).
The initial letters of 'usually triangular' is 'ut'.
'ut' inserted within 'class' is 'CUTLASS'.
(Other definitions for cutlass that I've seen before include "Short curved naval sword" , "Steel" , "Pirate sword" , "Pirate's sword" , "Short, broad-bladed sword" .)