Indecent number we'd revised (4)
I believe the answer is:
lewd
'indecent' is the definition.
('lewd' can be a synonym of 'indecent')
'number we'd revised' is the wordplay.
'number' becomes 'l' (L is an example).
'revised' indicates anagramming the letters.
'wed' anagrammed gives 'ewd'.
'l'+'ewd'='LEWD'
(Other definitions for lewd that I've seen before include "Salacious" , "blue" , "Indecent, licentious" , "Smutty" , "Sexually crude" .)