Used to be protected by pig and horse? Nonsense (7)
I believe the answer is:
hogwash
'nonsense' is the definition.
(thesaurus)
'used to be protected by pig and horse?' is the wordplay.
'used to be' becomes 'was' (I've seen this before).
'protected by' is an insertion indicator.
'pig' becomes 'hog' ('hog' can be a synonym of 'pig').
'and' means one lot of letters go next to another.
'horse?' becomes 'h' ('H' can be a synonym of 'horse').
'hog'+'h'='hogh'
'was' put into 'hogh' is 'HOGWASH'.
(Other definitions for hogwash that I've seen before include "Balderdash - pigswill" , "Piffle" , "Pigswill - nonsense!" , "Utter nonsense" , "Colloquial nonsense to pig cleanser" .)