Sign for a number in harbour (7)
I believe the answer is:
portent
'sign' is the definition.
(a portent is a sign of something)
'a number in harbour' is the wordplay.
'a number' becomes 'ten' (ten is a number).
'in' is an insertion indicator.
'harbour' becomes 'port' (I've seen this before).
'ten' going within 'port' is 'PORTENT'.
'for' is the link.
(Other definitions for portent that I've seen before include "Sign of thing to come" , "bad sign" , "Sign or warning of an event likely to happen" , "Prophetic sign" , "Sign, omen" .)