Badly underestimate mates leaving, having got together again (8)
I believe the answer is:
reunited
'together again' is the definition.
(I've seen this before)
'badly underestimate mates leaving' is the wordplay.
'badly' indicates an anagram (letters in the wrong order).
'leaving' indicates named letters should be taken away.
'underestimate' with 'mates' taken out is 'undreite'.
'undreite' anagrammed gives 'REUNITED'.
'having got' acts as a link.
(Other definitions for reunited that I've seen before include "Brought or joined together again" , "Brought together again after separation" , "made one again" , "Got together again" , "back with old friends?" .)