Outsider, criminal rounded on leader in gymkhana (8)
I believe the answer is:
underdog
'outsider' is the definition.
(I've seen this before)
'criminal rounded on leader in gymkhana' is the wordplay.
'criminal' indicates an anagram.
'on' means one lot of letters go next to another.
'leader in' indicates taking the first letters.
The first letter of 'gymkhana' is 'g'.
'rounded' anagrammed gives 'underdo'.
'underdo'+'g'='UNDERDOG'
(Other definitions for underdog that I've seen before include "Weaker contestant" , "outsider" , "Least favoured competitor" , "One of low status" , "One at a disadvantage" .)