Cracking clue about northern relative (5)
I believe the answer is:
uncle
'relative' is the definition.
(I know that relative can be written as uncle)
'cracking clue about northern' is the wordplay.
'cracking' indicates putting letters inside (inserted letters go into a gap or crack).
'about' indicates anagramming the letters.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'clue' is an anagram of 'ucle'.
'ucle' placed around 'n' is 'UNCLE'.
(Other definitions for uncle that I've seen before include "For example, Tom Cobbleigh" , "Grandfather's son" , "Family relation" , "Pawnbroker's nickname" , "older relative" .)