Sign of pressure thus internalised by current staff (6)
I believe the answer is:
isobar
'sign of pressure' is the definition.
(an isobar indicates pressure on a map)
'thus internalised by current staff' is the wordplay.
'thus' becomes 'so' (synonyms).
'internalised by' means one lot of letters goes inside another.
'current' becomes 'I' (symbol for electric current).
'staff' becomes 'bar' (both can mean a pole or stick).
'i'+'bar'='ibar'
'so' put within 'ibar' is 'ISOBAR'.
(Other definitions for isobar that I've seen before include "Meteorological line joining points of equal pressure" , "Line in front" , "depression indicator?" , "Line on map connecting areas of equal pressure" , "Line that connects places of equal air pressure" .)