Nobleman (a French one in bed?) (5)
I believe the answer is:
count
'nobleman' is the definition.
(count is a type of aristocrat)
'a french one in bed?' is the wordplay.
'a french one' becomes 'un' ('one' in French).
'in' indicates putting letters inside.
'bed?' becomes 'cot' (cot is a type of bed).
'un' put within 'cot' is 'COUNT'.
(Other definitions for count that I've seen before include "Nobleman - tell how many" , "Sum up" , "Matter to consider" , "regard" , "earl abroad" .)