Signs figure in harbour towns (8)
I believe the answer is:
portents
'signs' is the definition.
(I know this)
'figure in harbour towns' is the wordplay.
'figure' becomes 'ten' (I've seen this before).
'in' is an insertion indicator.
'harbour towns' becomes 'ports' (a port is a town with a harbour).
'ten' put within 'ports' is 'PORTENTS'.
(Other definitions for portents that I've seen before include "Signs of things about to happen" , "Omens" , "Ominous signs" .)