Slosh around a county bordering on Wales once (5)
I believe the answer is:
salop
'county bordering on wales once' is the definition.
(historical name for Shropshire)
'slosh around a' is the wordplay.
'slosh' becomes 'slop' (synonyms).
'around' is an insertion indicator.
'slop' placed around 'a' is 'SALOP'.
(Other definitions for salop that I've seen before include "Alternative name for the county of Shropshire" , "Shropshire in short" , "Shropshire (abbreviation)" , "Shropshire (abbr.)" , "Telford here" .)