Instinct one has to take a side (6)
I believe the answer is:
haunch
'side' is the definition.
(thesaurus)
'instinct one has to take a' is the wordplay.
I cannot really understand how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is located in the answer.
'a' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for haunch that I've seen before include "Leg and loin (of, eg, venison)" , "Loin and leg of a quadruped" , "joint of meat" , "Fleshy hindquarter of an animal" , "Buttock and thigh, taken together" .)