Spectral deity visiting London region (7)
I believe the answer is:
rainbow
'spectral' is the definition.
(a rainbow is a spectrum)
'deity visiting london region' is the wordplay.
'deity' becomes 'ra' (Ra is an example).
'visiting' becomes 'in'.
'london region' becomes 'bow' (area of London).
'ra'+'in'+'bow'='RAINBOW'
(Other definitions for rainbow that I've seen before include "Water-colour production" , "Natural multicoloured arc" , "Natural arch of colours" , "Arc of colour in the sky" , "Phenomenon produced by light passing through water droplets" .)