Out of a relationship, one side being hurt (5)
I believe the answer is:
ached
'being hurt' is the definition.
(I've seen this before)
'out of a relationship one side' is the wordplay.
'out of a relationship' becomes 'unattached' (no longer attached to someone).
'one side' indicates taking half.
'unattached' with half the letters taken is 'ACHED'.
(Other definitions for ached that I've seen before include "caused discomfort" , "Had a continuous pain" , "Felt a dull pain" , "Hurt" , "suffered" .)