Song is number one for unthinking people (8)
I believe the answer is:
airheads
'unthinking people' is the definition.
(I've seen this before)
'song is number one' is the wordplay.
'song' becomes 'air' (I've seen this before).
'is number one' becomes 'heads' (I can't explain this - if you can you should give a lot more credence to this answer).
'air'+'heads'='AIRHEADS'
'for' is the link.
Can you help me to learn more?
(Other definitions for airheads that I've seen before include "unthinking people" , "Feather-brains" , "Fools" , "Idiots" , "Dopes" .)