Cast one kept in outbuilding (5)
I believe the answer is:
shied
'cast' is the definition.
(cast can mean to throw or shy)
'one kept in outbuilding' is the wordplay.
'one' becomes 'i' (Roman numeral).
'kept in' is an insertion indicator.
'outbuilding' becomes 'shed' (outbuilding in a garden).
'i' placed within 'shed' is 'SHIED'.
(Other definitions for shied that I've seen before include "horse did this?" , "Jumped nervously; threw" , "Flung, tossed" , "Thrown" , "started" .)