Most costly sword picked up between two streets (8)
I believe the answer is:
steepest
'most costly' is the definition.
(steep can mean expensive)
'sword picked up between two streets' is the wordplay.
'two streets' can mean two replacements for 'street'.
'sword' becomes 'epee' (epee is a kind of sword).
'picked up' is a reversal indicator.
'between' is an insertion indicator.
'street' becomes 'st'.
'street' becomes 'st'.
'epee' reversed gives 'eepe'.
'st'+'st'='stst'
'eepe' inserted within 'stst' is 'STEEPEST'.
(Other definitions for steepest that I've seen before include "Rising most sharply" , "With the sharpest incline" , "extremely dear" , "Most expensive" , "most severe" .)