Glass or beer mat needs changing (9)
I believe the answer is:
barometer
'glass' is the definition.
(thesaurus)
'or beer mat needs changing' is the wordplay.
'needs changing' indicates anagramming the letters (I've seen this in other clues).
'or'+'beer'+'mat'='orbeermat'
'orbeermat' with letters rearranged gives 'BAROMETER'.
(Other definitions for barometer that I've seen before include "Instrument that measures atmospheric pressure" , "Air-pressure measurer" , "One measures pressure" , "It checks state of sky" , "Pressure indicator" .)