It's terrible - and not quite legitimate (5)
I believe the answer is:
awful
'it's terrible' is the definition.
(synonyms)
'not quite legitimate' is the wordplay.
'not quite' means to remove the first letter.
'legitimate' becomes 'lawful' (synonyms).
'lawful' with its first letter taken off is 'AWFUL'.
'and' acts as a link.
(Other definitions for awful that I've seen before include "Really terrible, ghastly" , "Horrible" , "Dreadful, really bad" , "rubbish" , "Not good at all" .)