Party, hard one to fit into schedule (7)
I believe the answer is:
roister
'party hard' is the definition.
'roister' can be an answer for 'party' (I've seen this before). I'm not sure about the 'hard' bit.
'one to fit into schedule' is the wordplay.
'one to fit' becomes 'i' (I can't explain this - if you can you should give a lot more credence to this answer).
'into' indicates putting letters inside.
'schedule' becomes 'roster' (I've seen this before).
'i' put into 'roster' is 'ROISTER'.
Can you help me to learn more?
(Other definitions for roister that I've seen before include "Behave boisterously" , "I rest or (anag) - engage in noisy merrymaking" , "riot" , "Carouse" , "bluster" .)