Hardline leader breaks up rubbish party (6)
I believe the answer is:
thrash
'party' is the definition.
(thrash can informally mean a party)
'hardline leader breaks up rubbish' is the wordplay.
'leader' suggests taking the first letters.
'breaks' indicates putting letters inside (inserted letters break into the word).
'up rubbish' becomes 'trash' (synonyms. I am not sure about the 'up' bit.).
The initial letter of 'hardline' is 'h'.
'h' put into 'trash' is 'THRASH'.
Can you help me to learn more?
(Other definitions for thrash that I've seen before include "Soundly defeat; party" , "Flog; party" , "Beat hard" , "worst > party" , "Whip repeatedly" .)