Some irrational relationship (5)
I believe the answer is:
ratio
'relationship' is the definition.
(I've seen this in another clue)
'some irrational' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'RATIO' is hidden in the letters of 'irrational'.
(Other definitions for ratio that I've seen before include "five to four, perhaps" , "Measure expressed as a proportion" , "nine-to-five, say" , "Proportion between two numbers" , "Proportional relation between two numbers" .)