Swift measures blocked by Left (5)
I believe the answer is:
fleet
'swift' is the definition.
(I know that swift can be written as fleet)
'measures blocked by left' is the wordplay.
'measures' becomes 'feet' (foot is a kind of measure).
'blocked by' is an insertion indicator (I've seen this in other clues).
'left' becomes 'l' (common abbreviation).
'feet' placed around 'l' is 'FLEET'.
(Other definitions for fleet that I've seen before include "Group of vehicles operating together, ships say" , "Fast moving" , "Swift; group of ships" , "Navy - quick" , "Quickly pass" .)