There is possibly nothing in that conjecture (8)
I believe the answer is:
theorise
'that conjecture' is the definition.
(I've seen this before)
'there is possibly nothing in' is the wordplay.
'possibly' indicates an anagram.
'nothing' becomes 'o' (looks like zero - 0).
'in' indicates putting letters inside.
'there'+'is'='thereis'
'thereis' is an anagram of 'therise'.
'therise' placed around 'o' is 'THEORISE'.
(Other definitions for theorise that I've seen before include "Form of hypothesis or conjecture" , "Conjecture, hypothesise" , "Suppose" , "Form view" , "Think up possible explanations" .)