What I resolutely composed is a love song (4,5,3,3)

I believe the answer is:

till there was you

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'a love song' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'what i resolutely composed' is the wordplay.
'composed' indicates an anagram.
'what'+'i'+'resolutely'='whatiresolutely'
'whatiresolutely' with letters rearranged gives 'TILL THERE WAS YOU'.

'is' acts as a link.

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