Say 'Just a theory' on a regular basis (5)
I believe the answer is:
utter
'say' is the definition.
(to utter something is to say or speak it)
'just a theory on a regular basis' is the wordplay.
'on a regular basis' indicates alternate letters.
'just'+'a'+'theory'='justatheory'
The alternate letters of 'justatheory' are 'UTTER'.
(Other definitions for utter that I've seen before include "Absolute - say" , "Dead" , "Speak or say" , "(Of counterfeit money) put into circulation - say" , "Say; total" .)