Rough National Trust walk ahead (8)
I believe the answer is:
strident
'rough' is the definition.
(I've seen this before)
'national trust walk ahead' is the wordplay.
'national trust' becomes 'nt'.
'walk' becomes 'stride' (striding is a kind of walking).
'ahead' is a charade indicator (letters next to each other).
'nt' put after 'stride' is 'STRIDENT'.
(Other definitions for strident that I've seen before include "With a loud, harsh voice" , "giving views forcefully" , "making a lot of noise" , "Tumultuous" , "harsh sound" .)