Frequent drinking you ultimately had reduced (10)
I believe the answer is:
attenuated
'reduced' is the definition.
(similar in meaning)
'frequent drinking you ultimately had' is the wordplay.
'frequent' becomes 'attend' (I've seen this before).
'drinking' is an insertion indicator (inserted letters are consumed or drunk).
'ultimately' says to take the final letters.
'had' becomes 'ate' (e.g. I had an ice cream).
The final letter of 'you' is 'u'.
'u'+'ate'='uate'
'attend' enclosing 'uate' is 'ATTENUATED'.
(Other definitions for attenuated that I've seen before include "weakened" , "Reduced in strength" , "Made thin" , "Made thin/thinner" , "Made thinner" .)