Sleep with old man who dominated Europe (8)
I believe the answer is:
napoleon
'who dominated europe' is the definition.
'sleep with old man' is the wordplay.
'sleep' becomes 'nap' (I've seen this before).
'with' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'man' becomes 'leon' (**).
'nap'+'o'+'leon'='NAPOLEON'
(Other definitions for napoleon that I've seen before include "Cake" , "In France, former sovereign" , "Emperor - brandy" , "Corporal and emperor" , "no longer tender" .)