Flag up string of characters in global search (4)
I believe the answer is:
slab
'flag' is the definition.
(flag can mean a stone slab)
'up string of characters in global search' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'string of characters in' indicates a hidden word.
'bals' is hidden within 'global search'.
'bals' in reverse letter order is 'SLAB'.
(Other definitions for slab that I've seen before include "Flat mass of stone" , "Piece of paving" , "Mortuary table" , "Broad, thick piece of wood, stone" , "Large block of stone or concrete" .)