Before eleven, say, you ate to get thin (9)
I believe the answer is:
attenuate
'get thin' is the definition.
(I've seen this before)
'before eleven say you ate' is the wordplay.
'before eleven say' becomes 'atten' (I can't explain this - if you can you should believe this answer much more).
'you' becomes 'u' (homophones).
'atten'+'u'+'ate'='ATTENUATE'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for attenuate that I've seen before include "Reduce in strength" , "Slender" , "Spin out" , "become weak" , "Reduce (a signal)" .)