Crossword Genius

Lengthen professional treatise (8)

Ross

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" .)

I've seen this clue in The Times.
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