Thought caller might be talking (7)
I believe the answer is:
guessed
'talking' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs can occasionally mean the same thing.
'thought caller might' is the wordplay.
'thought' becomes 'guess' (guess is a kind of thought).
'caller might' becomes 'ed' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'guess'+'ed'='GUESSED'
'be' is the link.
Can you help me to learn more?
(Other definitions for guessed that I've seen before include "Reckoned, judged" , "Thought" , "was a speculator" , "perhaps got in one" , "Made estimate or hazarded opinion" .)