Aunt's theory is occasionally thorough (5)
I believe the answer is:
utter
'thorough' is the definition.
(thesaurus)
'aunt's theory is occasionally' is the wordplay.
'is occasionally' means one should take alternating letters.
'aunts'+'theory'='auntstheory'
The alternating letters of 'auntstheory' are 'UTTER'.
(Other definitions for utter that I've seen before include "Prize" , "To say, articulate" , "Make (a sound); complete" , "Perfect" , "Speak - it's extreme" .)