Kind of knife with a curve, that is (5)
I believe the answer is:
bowie
'kind of knife' is the definition.
'with a curve that is' is the wordplay.
'with' means one lot of letters go next to another.
'a curve' becomes 'bow' (bow is a kind of curve).
'that is' becomes 'ie' (synonyms).
'bow' next to 'ie' is 'BOWIE'.
(Other definitions for bowie that I've seen before include "Rock-based chameleon" , "Art rocker David" , "David --, singer" , "Dagger-knife" , "American frontiersman, killed at the Alamo, 1836" .)