Reformed TUC's plea close to arguable conjecture (9)
I believe the answer is:
speculate
'conjecture' is the definition.
('speculate' can be a synonym of 'conjecture')
'reformed tuc's plea close to arguable' is the wordplay.
'reformed' indicates an anagram.
'close to' says to take the final letters (the close/ending of).
The final letter of 'arguable' is 'e'.
'tucs'+'plea'='tucsplea'
'tucsplea' with letters rearranged gives 'speculat'.
'speculat'+'e'='SPECULATE'
(Other definitions for speculate that I've seen before include "Form opinion without definite knowledge" , "Buy in hope of profit, but with risk of loss" , "Make conjectures" , "Theorise without firm evidence" , "As before, observe" .)