Shed slender pair's vocalisation (4-2)
I believe the answer is:
lean-to
'shed' is the definition.
(I've seen this before)
'slender pair's vocalisation' is the wordplay.
'slender' becomes 'lean' (similar in meaning).
'pair's vocalisation' becomes 'to' (I can't justify this - if you can you should believe this answer much more).
'lean'+'to'='LEAN-TO'
Can you help me to learn more?
(Other definitions for lean-to that I've seen before include "Outhouse" , "Attached outbuilding" , "Type of shed" , "Building put up against another" , "Shed built against wall" .)