A couple that's overly outspoken (3)
I believe the answer is:
two
'a couple' is the definition.
(two people)
'overly outspoken' is the wordplay.
'overly' becomes 'too' (to too great a degree).
'outspoken' indicates a 'sounds like' (homophone) clue (as the word might be spoken).
'too' sounds like 'TWO'.
'that's' is the link.
(Other definitions for two that I've seen before include "II" , "'It takes ? to tango'" , "2" , "A low number" , "Couple, brace" .)