They hold an alternative opinion (6)
I believe the answer is:
theory
'opinion' is the definition.
(thesaurus)
'they hold an alternative' is the wordplay.
'hold' indicates putting letters inside.
'an alternative' becomes 'or'.
'they' placed around 'or' is 'THEORY'.
(Other definitions for theory that I've seen before include "One with specialised knowledge" , "Hypothesis - doctrine" , "Idea to explain something, say evolution" , "Explanation or system of anything" , "Supposition, hypothesis" .)