Forthright six-footer by ditch (9)
I believe the answer is:
trenchant
'forthright' is the definition.
(I've seen this before)
'six-footer by ditch' is the wordplay.
'six footer' becomes 'ant'.
'by' means one lot of letters go next to another.
'ditch' becomes 'trench' (trench is a kind of ditch).
'ant' put after 'trench' is 'TRENCHANT'.
(Other definitions for trenchant that I've seen before include "Distinctly defined" , "Forthright, vigorous" , "Fierce" , "Showing keenness" , "Cutting, forthright" .)