Being hasty is blow to career (8)
I believe the answer is:
slapdash
'being hasty' is the definition.
(I've seen this before)
'blow to career' is the wordplay.
'blow' becomes 'slap' (slap is a kind of blow).
'to career' becomes 'dash' (both can mean to move chaotically).
'slap'+'dash'='SLAPDASH'
'is' is the link.
(Other definitions for slapdash that I've seen before include "Quick, careless" , "Carelessly hurried" , "Perfunctory" , "Hasty and careless - hit and run?" , "Irresponsible" .)