Insect flying without left wing catches head on hatstand (5)
I believe the answer is:
aphid
'insect' is the definition.
(aphid is a kind of insect)
'flying without left wing catches head on hatstand' is the wordplay.
'flying' becomes 'rapid' (both can mean fast).
'without left wing' means to remove the first letter (without the letter on the left end of the word).
'catches' means one lot of letters goes inside another.
'head on' suggests taking the first letters.
The initial letter of 'hatstand' is 'h'.
'rapid' with its first letter taken off is 'apid'.
'apid' enclosing 'h' is 'APHID'.
(Other definitions for aphid that I've seen before include "little sucker" , "insect pest" , "Greenfly etc" , "Small plant-sucking insect" , "Insect feeding on plant juices" .)