Reason partner (5)
I believe the answer is:
rhyme
'reason partner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rhyme that I've seen before include "Box and cox, for example" , "Verse of poetry" , "Sound like" , "Poem, eg for nursery" , "Be like 'cat' and 'mat'" .)