Show a harbour light (7)
I believe the answer is:
portray
'show' is the definition.
(to portray is to show or depict)
'a harbour light' is the wordplay.
'a harbour' becomes 'port' (I've seen this before).
'light' becomes 'ray' (ray is a kind of light).
'port'+'ray'='PORTRAY'
(Other definitions for portray that I've seen before include "Depict in art" , "Use words to describe" , "Play" , "sketch" , "Paint a picture" .)