Unhappy singleton really on the loose! (6,5)

I believe the answer is:

lonely heart

'unhappy singleton' is the definition.

'really on the loose' is the wordplay.
'loose' indicates an anagram (letters are loosened from their original positions).
'really'+'on'+'the'='reallyonthe'
'reallyonthe' with letters rearranged gives 'LONELY HEART'.

(Another definition for lonely heart that I've seen is " One seeking love".)

I've seen this clue in The Telegraph.

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