Sleep with old Leon: he dominated Europe (8)
I believe the answer is:
napoleon
'he dominated europe' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'sleep with old leon' is the wordplay.
'sleep' becomes 'nap' (a nap is a short sleep).
'with' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'nap'+'o'+'leon'='NAPOLEON'
Can you help me to learn more?
(Other definitions for napoleon that I've seen before include "Solo, perhaps" , "card game" , "A pig farmer was chased by" , "Old French coin" , "Ruler" .)