Initially one shilling to enter Elizabeth I's haunt? (6)
I believe the answer is:
obsess
'haunt?' is the definition.
(to haunt can mean to cause an obsession)
'initially one shilling to enter elizabeth i's' is the wordplay.
'initially' indicates taking the first letters.
'shilling' becomes 's'.
'to enter' indicates putting letters inside.
'elizabeth i' becomes 'bess' (nickname as in Good Queen Bess).
The first letter of 'one' is 'o'.
's' put within 'bess' is 'bsess'.
'o'+'bsess'='OBSESS'
(Other definitions for obsess that I've seen before include "Prey on" , "Control" , "Bosses (anag.)" , "Preoccupy completely" , "Think only" .)