Worker south of fortified area is incisive (9)
I believe the answer is:
trenchant
'incisive' is the definition.
(I've seen this in another clue)
'worker south of fortified area' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'south of' is a charade indicator (letters next to each other) (in a down clue, some letters go under others).
'fortified area' becomes 'trench' (as in trench warfare).
'ant' after 'trench' is 'TRENCHANT'.
'is' is the link.
(Other definitions for trenchant that I've seen before include "Cutting, forthright" , "Distinctly defined" , "Clear" , "biting" , "Vigorous, incisive" .)