After reflection, one really enjoyed eating old cheese (5)
I believe the answer is:
gouda
'cheese' is the definition.
(Gouda cheese is an example)
'after reflection one really enjoyed eating old' is the wordplay.
'after reflection' shows that the letters should be reversed in order.
'one' becomes 'a'.
'really enjoyed' becomes 'dug' (dig can mean to really enjoy or appreciate).
'eating' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'a'+'dug'='adug'
'adug' placed around 'o' is 'aduog'.
'aduog' reversed gives 'GOUDA'.
(Other definitions for gouda that I've seen before include "Mild Dutch cheese" , "Round Dutch cheese" , "type of cheese" , "It comes from Holland" , "Dutch town" .)