Snail won - terribly relative (3-2-3)
I believe the answer is:
son-in-law
'relative' is the definition.
(son-in-law is a kind of relative)
'snail won terribly' is the wordplay.
'terribly' indicates an anagram.
'snail'+'won'='snailwon'
'snailwon' anagrammed gives 'SON-IN-LAW'.
(Other definitions for son-in-law that I've seen before include "Relative by marriage" , "Member of extended family" , "He's married to your daughter" , "Daughter's husband" .)