Useless pilot’s first collision (4)
I believe the answer is:
bump
'collision' is the definition.
(thesaurus)
'useless pilot's first' is the wordplay.
'useless' becomes 'bum' (I've seen this in other clues).
'first' indicates taking the first letters.
The first letter of 'pilots' is 'p'.
'bum'+'p'='BUMP'
(Other definitions for bump that I've seen before include "Knock - collide" , "Light blow or jolting collision" , "Collide with force or violence" , "striking" , "Impact injury" .)