Dicky, grounded little man? (8)
I believe the answer is:
'little man?' is the definition.
(an underdog is a weaker or smaller competitor)
'dicky grounded' is the wordplay.
'dicky' indicates an anagram (dicky can mean ill or non-functioning).
'grounded' is an anagram of 'UNDERDOG'.
(Other definitions for underdog that I've seen before include "One at a disadvantage", "One of low status", "Competitor expected to lose", "No drudge (anag.)", "Likely loser".)