A controversy brewing in glasshouse (12)
I believe the answer is:
conservatory
'glasshouse' is the definition.
(conservatory is a kind of glasshouse)
'a controversy brewing in' is the wordplay.
'brewing' is an anagram indicator.
'in' is an insertion indicator.
'controversy' with letters rearranged gives 'conservtory'.
'a' going into 'conservtory' is 'CONSERVATORY'.
(Other definitions for conservatory that I've seen before include "Hothouse" , "school of music" , "Sun lounge" .)