Mostly nice room tidied up for Greek character (7)
I believe the answer is:
omicron
'greek character' is the definition.
(letter from the Greek alphabet)
'mostly nice room tidied up' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'tidied up' is an anagram indicator.
'nice' with its final letter taken away is 'nic'.
'nic'+'room'='nicroom'
'nicroom' is an anagram of 'OMICRON'.
'for' acts as a link.
(Other definitions for omicron that I've seen before include "Round character" , "Greek character" , "character in Sophocles" , "Greek short 'o'" , "Foreign letter -- I'm no roc (anag)" .)