Hit the first speed trap (5)
I believe the answer is:
thump
'hit' is the definition.
(thumping is a kind of hitting)
'the first speed trap' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'first speed trap' becomes 'hump' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
't'+'hump'='THUMP'
Can you help me to learn more?
(Other definitions for thump that I've seen before include "Punch" , "Smack" , "(Sound of) heavy blow" , "Hit forcefully" , "Whack" .)