Within month, see one becoming tyrannical (8)
I believe the answer is:
despotic
'tyrannical' is the definition.
('despotic' can be a synonym of 'tyrannical')
'within month see one' is the wordplay.
'within' is an insertion indicator.
'month' becomes 'dec' (Dec is an example).
'see' becomes 'spot' (I've seen this before**).
'one' becomes 'i' (Roman numeral).
'spot'+'i'='spoti'
'dec' placed around 'spoti' is 'DESPOTIC'.
'becoming' is the link.
(Other definitions for despotic that I've seen before include "Dictatorial" , "Referring to absolute power" , "of tyrant" , "Autocratic" , "High-handed" .)