Many a vehicle may serve for sleeping (5)
I believe the answer is:
divan
'serve for sleeping' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'many a vehicle' is the wordplay.
'many' becomes 'D' (Roman numeral for 500).
'a vehicle' becomes 'ivan' (I can't justify this - if you can you should give a lot more credence to this answer).
'd'+'ivan'='DIVAN'
'may' is the link.
Can you help me to learn more?
(Other definitions for divan that I've seen before include "resting place" , "Viand (anag.)" , "Piece of furniture" , "Sideless, backless couch" , "Sofa - bed" .)