Set about a Highland Scot (4)
I believe the answer is:
gael
'highland scot' is the definition.
(I've seen this before)
'set about a' is the wordplay.
'set' becomes 'gel' (both can mean to harden).
'about' indicates putting letters inside.
'gel' enclosing 'a' is 'GAEL'.
(Other definitions for gael that I've seen before include "Scottish highlander" , "Highland Scot" , "Irish speaker?" , "Celt of Ireland, the Scottish Highlands and the IOM" , "Celtic language speaker" .)