Frontline target (4)
I believe the answer is:
flea
'frontline target' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for flea that I've seen before include "Little jumping insect" , "Tiny circus performer?" , "Member of a circus?" , "Small blood-sucking insect" , "Wingless, jumping insect" .)