Lengthen professional treatise (8)
I believe the answer is:
protract
'lengthen' is the definition.
(protracting is a kind of lengthening)
'professional treatise' is the wordplay.
'professional' becomes 'pro' (synonyms).
'treatise' becomes 'tract' (tract is a kind of treatise).
'pro'+'tract'='PROTRACT'
(Other definitions for protract that I've seen before include "Draw out, prolong" , "Extend length" , "Increase" , "Draw out, lengthen the time" , "Drag out at length" .)