Bill on immigration's first, or later? (9)
I believe the answer is:
posterior
'later?' is the definition.
(thesaurus)
'bill on immigration's first or' is the wordplay.
'bill' becomes 'poster' ('poster' can be a synonym of 'bill'**).
'on' says to put letters next to each other.
'first' suggests taking the first letters.
The initial letter of 'immigrations' is 'i'.
'poster'+'i'+'or'='POSTERIOR'
(Other definitions for posterior that I've seen before include "Backside" , "Rear" , "later" , "Stern" , "Further back; bottom" .)