Having let one in, became chatty (6,2)
I believe the answer is:
opened up
'chatty' 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.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for opened up that I've seen before include "made disclosure" , "started talking" , "started shooting" , "Poured heart out" , "Offered a freer view" .)