Irregular stuff inspires core of nationalist party (6)
I believe the answer is:
random
'irregular' is the definition.
(I know that irregular can be written as random)
'stuff inspires core of nationalist party' is the wordplay.
'stuff' becomes 'ram' (ram can mean to stuff or push).
'inspires' is an insertion indicator (inspire means to breathe in).
'core of nationalist' becomes 'n' (I can't justify this - if you can you should give a lot more credence to this answer).
'party' becomes 'do' (do can informally mean a party).
'n'+'do'='ndo'
'ram' going around 'ndo' is 'RANDOM'.
Can you help me to learn more?
(Other definitions for random that I've seen before include "Without a particular aim or purpose - say a choice" , "Lacking any definite plan, order or purpose" , "Lacking aim or method, haphazard" , "without any plan" , "Done or taken without particular aim" .)