Somehow relating to constituent (8)
I believe the answer is:
integral
'to constituent' is the definition.
(I've seen this before)
'somehow relating' is the wordplay.
'somehow' indicates an anagram.
'relating' is an anagram of 'INTEGRAL'.
(Other definitions for integral that I've seen before include "A component and indispensable part of, like triangle" , "Fundamental; function" , "Triangle/relating/altering/alerting (anag)" , "Altering so as to be necessary to the whole" , "Built-in -- 1 across (anag)" .)