First-class but 1000 deducted for being without purpose (7)
I believe the answer is:
aimless
'purpose' is the definition.
(I have seen 'Lacking purpose' mean 'aimless' so perhaps 'purpose' could also mean 'aimless')
'first-class but 1000 deducted for being without' is the wordplay.
'first-class but a000 deducted for being' becomes 'aim' (I am not sure about this - if you are sure you should believe this answer much more).
'without' becomes 'less' (I've seen this in another clue).
'aim'+'less'='AIMLESS'
Can you help me to learn more?
(Other definitions for aimless that I've seen before include "without sense of purpose" , "Without direction or ambition" , "Without objectives" , "at a loose end" , "Haphazard" .)