Young new mum, irate (8)
I believe the answer is:
immature
'young' is the definition.
('immature' can be a synonym of 'young')
'new mum irate' is the wordplay.
'new' indicates anagramming the letters.
'mum'+'irate'='mumirate'
'mumirate' with letters rearranged gives 'IMMATURE'.
(Other definitions for immature that I've seen before include "Not fully ripe or developed" , "Like childish behaviour" , "Lacking in development" , "Not yet adult or fully developed" , "Undeveloped - unripe" .)