Head of state's love a Shakespearean queen rejected (5)
I believe the answer is:
obama
'head of state's' is the definition.
(former US president Barack Obama)
'love a shakespearean queen rejected' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'shakespearean queen' becomes 'Mab' (Queen Mab is mentioned in Romeo and Juliet).
'rejected' says the letters should be written in reverse.
'a'+'mab'='amab'
'amab' in reverse letter order is 'bama'.
'o'+'bama'='OBAMA'
(Other definitions for obama that I've seen before include "44th US President" , "US presidential candidate" , "President awarded the 2009 Nobel Peace Prize" , "Politician" , "Important figure" .)