Single into middle of Rhyl's dear (5)
I believe the answer is:
honey
'dear' is the definition.
('honey' can be a synonym of 'dear')
'single into middle of rhyl's' is the wordplay.
'single' becomes 'one' (I've seen this before).
'into' indicates putting letters inside.
'middle of' says to take the centre.
The middle letters of 'rhyl' are 'hy'.
'one' placed inside 'hy' is 'HONEY'.
(Other definitions for honey that I've seen before include "Darling; hive product" , "Hive fluid" , "Sweet yellow liquid" , "Food from hive" , "Sweet term of endearment" .)