Rook — mouldy thing with loathsome wings (6)
I believe the answer is:
castle
'rook' is the definition.
(synonyms)
'mouldy thing with loathsome wings' is the wordplay.
'mouldy thing' becomes 'cast' (a cast is something used as a mould).
'with' is a charade indicator (letters next to each other).
'wings' means to remove the middle letters (the wings or edge letters of the word are used).
'loathsome' with its centre removed is 'le'.
'cast'+'le'='CASTLE'
(Other definitions for castle that I've seen before include "Left's Social Services secretary" , "Move rook and king in the same turn" , "fortification" , "man of 1d [ADVENTURE]" , "eg Edinburgh or Windsor" .)