Legitimate to lose head, it's terrible (5)
I believe the answer is:
awful
'terrible' is the definition.
(I know that terrible can be written as awful)
'legitimate to lose head' is the wordplay.
'legitimate' becomes 'lawful' (synonyms).
'to lose head' means to remove the first letter (lose (remove) the head (start) of the word).
'lawful' with its initial letter removed is 'AWFUL'.
'it's' acts as a link.
(Other definitions for awful that I've seen before include "Horrible" , "shocking" , "Exceptionally bad" , "Disgusting" , "Extremely bad or unpleasant" .)