Keep clear of a vacuum (5)
I believe the answer is:
avoid
'keep clear of' is the definition.
(I've seen this in another clue)
'a vacuum' is the wordplay.
'vacuum' becomes 'void' (I've seen this in another clue).
'a'+'void'='AVOID'
(Other definitions for avoid that I've seen before include "Shy away from" , "Stay clear of" , "Prevent from happening; nullify (law)" , "Refrain from (something)" , "Get out of" .)