Make rare gaffe initially being an outsider (8)
'an outsider' is the definition.
(I've seen this before)
'make rare gaffe initially' is the wordplay.
I cannot quite see how this works, but
'make' could be 'do' (synonyms) and 'do' is found within the answer.
'rare' could be 'r' (listed by Oxford Dictionary of Abbreviations) and 'r' is found within the answer.
This explanation may well be incorrect...
'being' is the link.
Can you help me to learn more?
(Other definitions for underdog that I've seen before include "outsider", "certainly not a favourite!", "No drudge (anag.)", "Less fancied competitor", "one not expected to win".)