One ideally suited to another (4,4)
(Another definition for soulmate that I've seen is " Person ideally suited to another".)
'one ideally suited to another' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for soul mate that I've seen before include "Person ideally suited to another" , "Person sharing deep affinity" , "Friend with a deeply personal relationship, like spiritual spouse" , "One's ideal partner" , "One ideally suited to another" .)