Worthless pilot's first collision (4)
I believe the answer is:
bump
'collision' is the definition.
(I've seen this before)
'worthless pilot's first' is the wordplay.
'worthless' becomes 'bum' (I've seen this in another clue).
'pilot's first' becomes 'p' (1st letter of 'pilot').
'bum'+'p'='BUMP'
(Other definitions for bump that I've seen before include "Impact injury" , "Collide - protuberance" , "Collide with force or violence" , "Bash into" , "Thud down, into" .)