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' is an anagram of 'BAROMETER'.
(Other definitions for barometer that I've seen before include "It checks state of sky" , "Pressure gauge" , "It measures air pressure" , "Weather forecaster" , "Atmospheric pressure measurer" .)