Its glue dissolved, becoming most hideous (7)
I believe the answer is:
ugliest
'most hideous' is the definition.
('ugly' can be similar in meaning to 'hideous')
'its glue dissolved' is the wordplay.
'dissolved' indicates an anagram.
'its'+'glue'='itsglue'
'itsglue' anagrammed gives 'UGLIEST'.
'becoming' is the link.
(Other definitions for ugliest that I've seen before include "Least attractive" , "Most displeasing to the senses" , "Most deficient in beauty" , "Most horrible looking" , "superlatively unattractive" .)