20 across: For a northerner, it's great to be back! (8)
?H?M?I?N (champion? if so why?)
50 across: Umnusual in a sea to flow back into river (8)
?E?E???A
47 down: cold cntinent including singular source of senna (6) ???S?A
Pretty sure this is CASSIA
46 down: Unconvincing books bring complaint (6)
???E?T
Sick of looking at it now so help much appreciated