City visit involves son in great delight (7)
I believe the answer is:
ecstasy
'great delight' is the definition.
(I've seen this before)
'city visit involves son' is the wordplay.
'city' becomes 'EC' (the City of London postcode).
'visit' becomes 'stay' (I've seen this before).
'involves' is an insertion indicator.
'son' becomes 's' (genealogical abbreviation for son).
'stay' placed around 's' is 'stasy'.
'ec'+'stasy'='ECSTASY'
'in' is the link.
(Other definitions for ecstasy that I've seen before include "Feeling of great rapture" , "State of bliss or euphoria" , "Great pleasure" , "transports" , "Overwhelming joy" .)