Rattling of triangle necessary? (8)
I believe the answer is:
integral
'necessary?' is the definition.
(I've seen this before)
'rattling of triangle' is the wordplay.
'rattling of' indicates an anagram (letters rattle around).
'triangle' is an anagram of 'INTEGRAL'.
(Other definitions for integral that I've seen before include "Of part necessary to the completeness of a whole" , "Belonging as an essential part" , "Relating to being complete" , "Forming part of the whole" , "though whole" .)