Is no longer soft, and may sting (4)
I believe the answer is:
wasp
'may sting' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'is no longer soft' is the wordplay.
'is no longer' becomes 'was' (eg if someone 'was' sad, they're no longer sad).
'soft' becomes 'p' (musical abbreviation for piano, 'soft').
'was'+'p'='WASP'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for wasp that I've seen before include "Insect with a formidable sting" , "Social worker" , "Flyer" , "Insect often irritating in summer" , "Insect; thin (waist)" .)