Uniquely large rear end of pop diva (4)
I believe the answer is:
once
'uniquely' is the definition.
(once can mean one time or uniquely)
'end of pop diva' is the wordplay.
'end of' says to hollow out the word (remove centre letters) (I've seen 'ends' mean this).
'pop' becomes 'oldman' (synonyms).
'diva' becomes 'ce' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'oldman' with its centre removed is 'on'.
'on'+'ce'='ONCE'
'large rear' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
(Other definitions for once that I've seen before include "yesterday" , "Cone (anag.)" , "in bygone days" , "Formerly; as soon as" , "Quick examination" .)