Immature, year-old gnu heading north (5)
I believe the answer is:
young
'immature' is the definition.
('young' can be a synonym of 'immature')
'year-old gnu heading north' is the wordplay.
'year' becomes 'y'.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'heading north' says the letters should be written backwards.
'gnu' backwards is 'ung'.
'y'+'o'+'ung'='YOUNG'
(Other definitions for young that I've seen before include "In early life" , "Mormon leader" , "Inexperienced" , "Newly-developed" , "interviewer of PMs on the radio" .)