Slowly describing a type of shed (4-2)
I believe the answer is:
lean-to
'type of shed' is the definition.
(I've seen this before)
'slowly describing a' is the wordplay.
'slowly' becomes 'lento' (synonyms).
'describing' is an insertion indicator (describe can mean to form the outline of).
'lento' going around 'a' is 'LEAN-TO'.
(Other definitions for lean-to that I've seen before include "Building built against another" , "Outbuilding propped against another wall" , "Rough shelter lying against the wall of a building" , "It's roof slants against a wall" , "Small building attached to larger one" .)