Pier halved speed of corsair (6)
I believe the answer is:
pirate
'corsair' is the definition.
(I've seen this before)
'pier halved speed' is the wordplay.
'halved' indicates taking half.
'speed' becomes 'rate' (the speed at which something happens).
'pier' cut in half is 'pi'.
'pi'+'rate'='PIRATE'
'of' acts as a link.
(Other definitions for pirate that I've seen before include "Rover" , "He sailed under the Jolly Roger" , "Penzance marauder ?" , "Silver, perhaps" , "sailor" .)