Possible pet guarding old outdoor garment (4)
I believe the answer is:
coat
'outdoor garment' is the definition.
'possible pet guarding old' is the wordplay.
'possible pet' becomes 'cat'.
'guarding' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'cat' going around 'o' is 'COAT'.
(Other definitions for coat that I've seen before include "Outer garment" , "Warm outerwear" , "Jacket" , "Fleece" , "Apply layer" .)