Horse turned irrational, returning fruit (7)
I believe the answer is:
rosehip
'fruit' is the definition.
(fruit of a rose plant)
'horse turned irrational returning' is the wordplay.
'turned' indicates an anagram.
'irrational' becomes 'pi' (an irrational number).
'returning' is a reversal indicator.
'pi' in reverse letter order is 'ip'.
'horse' is an anagram of 'roseh'.
'roseh'+'ip'='ROSEHIP'
(Other definitions for rosehip that I've seen before include "Fruit of a popular flower" , "Wild fruit for syrup" , "Part of plant" , "Wayside fruit" , "Fruit from flower providing syrup" .)