Straight path's ending below (5)
I believe the answer is:
neath
'below' is the definition.
(thesaurus)
'straight path's ending' is the wordplay.
'straight' becomes 'neat' ('neat' can be a synonym of 'straight').
'ending' suggests the final letters.
The final letter of 'path' is 'h'.
'neat'+'h'='NEATH'
(Other definitions for neath that I've seen before include "Welsh team" , "River" , "Town near Swansea" , "Welsh industrial town" , "not on top" .)