Namely, he gets excited about marriage (8)
I believe the answer is:
hymeneal
'marriage' is the definition.
(thesaurus)
'namely he gets excited' is the wordplay.
'gets excited' is an anagram indicator.
'namely'+'he'='namelyhe'
'namelyhe' with letters rearranged gives 'HYMENEAL'.
'about' acts as a link.
(Other definitions for hymeneal that I've seen before include "about the match?" , "celebrating marriage" , "Concerning marriage" .)