Bats metabolise without oxygen in August (9)
I believe the answer is:
estimable
'august' is the definition.
(thesaurus)
'bats metabolise without oxygen' is the wordplay.
'bats' is an anagram indicator (slang for mad).
'without' indicates named letters should be taken away.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'metabolise' with 'o' taken away is 'metablise'.
'metablise' anagrammed gives 'ESTIMABLE'.
'in' acts as a link.
(Other definitions for estimable that I've seen before include "Deserving respect" , "Highly valued" , "respected" , "Worthy of great respect" , "highly regarded" .)