Space behind a duck (5)
I believe the answer is:
avoid
'duck' is the definition.
(duck can mean to avoid)
'space behind a' is the wordplay.
'space' becomes 'void' (void is a kind of space).
'behind' means one lot of letters go next to another.
'void' put after 'a' is 'AVOID'.
(Other definitions for avoid that I've seen before include "Desist" , "Keep away from (someone/something)" , "Fend off" , "Sidestep" , "Steer clear of, dodge" .)